University  of  California  •  Berkeley 


DEPARTMENT  OF  THE  INTERIOR-U.  S.  GEOLOGICAL  SURVEY 
CHARLES  I).  WALCOTT,  DIRECTOR 


GEORGE   HOMA1STS   ELDRIDG-E 


EXTRACT  FKOM  THE  SIXTEENTH  ANNUAL  REPORT  OF  THE  SURVEY,  1894-95 
PART  IT-PAPERS  OK  AN'  ECONOMIC  CHARACTER 


\\  A  s  ii  i  N  <; 

GOVERNMENT     PRINTING     OFFICE 

1895 


A  GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 


GEORGE    H.    ELDRIDGE. 


211 


CONTENTS. 


Page. 

Prefatory 217 

Topography 217 

Drainage  system  of  the  Snake  River 217 

Drainage  system  of  the  Columbia  River 218 

Mountains 219 

Canyons  anil  iutermontaue  valleys 220 

Glacial  action 223 

The  formations 224 

Granites  and  metamorphic  rocks 224 

Archean 224 

Algonkian 225 

Unaltered  sedimentary  rocks 226 

Paleozoic 226 

Cenozoic 230 

Pleistocene 234 

Eruptive  rocks 234 

General  structural  features 948 

Mining  districts 250 

Gold  and  silver 250 

Bear  Creek  district 250 

Atlanta  district 253 

Sheep  Mountain  district 258 

Yellow  Jacket  district 259 

Wood  River  district 264 

Silver  City  district 271 

Placers 273 

Coal 274 

Salmon  City  Valley 274 

Payette  Valley 274 

Agriculture 275 

213 


}i  A,  y\ ' 


ILLUSTRATIONS. 


Page 

PLATE     XV.  General  map  of  the  State  of  Idaho 216 

XVI.  Sketch-map  of  Atlanta  district 254 

XVII.  Skotch-map  of  Wood  Rivet  district 264 

FIG.  38.  Section  of  Tertiary  gravels  at  Lemhi  placer  mine 233 

39.  Sketch-map  of  Bear  Creek  district 251 

40.  Sketch-map  of  Yellow  Jacket  district 260 

41.  Sketch-map  of  Silver  City 272 

215 


U.   S.   GEOLOGICAL  SURVEY 


GENERAL  MAP  OF 


SIXTEENTH  ANNUAL  REPORT      PART  II      PL.    XV 


STATE  OF  IDAHO. 


A  GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 


BY  GEOKGE  H.  ELDEIDGE. 


PREFATORY. 

The  following  pages  contain  an  account  of  a  reconnaissance  across 
Idaho  on  a  northeast  line  through  Boise  and  Salmon  City.  The  work 
was  authorized  as  a  preliminary  to  the  future  geologic  study  of  the 
State.  In  its  prosecution  the  available  maps  have  been  the  atlas  sheets 
of  the  Survey,  between  the  meridians  of  114°  and  117°  west  and  the 
parallels  of  43°  and  44°  30'  north;  the  maps  of  the  United  States  Gen- 
eral Land  Office;  and  a  rough  sketch-map  of  Lemhi  County.1  The 
general  map  of  the  State  accompanying  this  report  is  a  compilation 
from  the  above  sources. 

The  fossil  plant  remains  referred  to  in  the  text  have  been  identified 
by  Mr.  Knowlton ;  the  mollusks,  by  Messrs.  Walcott  and  Stanton ;  the 
eruptives,  by  Mr.  Cross ;  and  the  chemical  analyses  have  been  made  in 
the  laboratory  of  the  Survey. 

TOPOGRAPHY. 

The  broad  topographic  features  of  Idaho  are  the  drainage  systems  of 
the  Snake  and  Columbia  rivers,  with  a  vast  arid  plain  along  the  former 
stream;  a  labyrinthine  mass  of  rugged  mountains  northward  from  the 
plain;  and  a  succession  of  desert  ranges  on  the  divide  between  the 
Snake  Kiver  and  the  Great  Basin,  along  the  southern  border  of  the 
State. 

DRAINAGE    SYSTEM   OF   THE    SNAKE    RIVER. 

The  Snake  Plains,  which  extend  completely  across  the  southern  end 
of  Idaho,  constitute  one  of  the  prominent  features  of  the  West.  The 
floor  is  rolling,  consisting  of  sand  and  lava  underlain  by  Tertiary  sedi- 
ments several  hundred  feet  thick.  These  have  been  cut  by  the  river  to 
a  depth  of  400  to  1,000  feet,  the  stream  being  confined  between  canyon 
walls,  or  lying  in  a  valley  with  bottom-lands  on  either  side.  Following 
are  a  few  of  the  determined  altitudes  of  the  plains.2 

1  Prepared  by  Mr.  Birdseye,  Salmon  City,  Idaho. 

'  Chiefly  from  GanueU's  Dictionary  of  Altitudea,  Bull.  U.  S.  Geol.  Survey  No.  70,  1891. 

217 


218 


GEOLOGICAL    RECONNAISSANCE    ACROSS    IDAHO. 


Altitudes  of  localities  on  the  Snake  Plains. 


Feet. 

Slioshone 3,975 

Bliss 3, 269 

Glenns  Ferry 2, 556 

Boise 2,768 

Kaiupa 2, 489 

Caldwell 2,  374 

Weiser 2, 125 


Feet. 

East  end  of  Snake  Plains,  about  . .  5, 000 

Camas 4,822 

Market  Lake 4, 781 

Eagle  Rock 4,714 

Blackibot 4,  505 

Pocatello 4,  468 

American  Falls 4, 343 

Minidoka 4,  287 

These  figures  show  a.  gentle  gradient  of  approximately  3,000  feet  in 
400  miles,  or  an  average  of  7£  feet  to  the  mile,  a  little  heavier  at  the 
eastern  end,  a  little  lighter  at  the  western. 

Additional  elevations  along  Snake  River  are:  Lewiston,  about  1,100 
feet;  Pasco  Junction,  Wash.,  386  feet;  Snake  Eiver  (mouth),  328  feet. 
These  indicate  the  maintenance,  for  the  balance  of  the  river's  course, 
from  Weiser  to  the  junction  with  the  Columbia,  of  a  gradient  of 
approximately  7  feet. 

The  drainage  of  Idaho,  with  the  exception  of  the  southeast  corner 
and  a  narrow  strip  of  130  miles  at  the  north,  is  entirely  through  the 
Snake  Kiver.  The  copious  waters  of  this  stream  are  derived  from  the 
Continental  Divide  in  the  vicinity  of  the  Yellowstone  Park,  from  the 
great  mass  of  mountains  north,  and  from  the  divide  between  it  and 
the  Salt  Lake  and  Humboldt  regions  to  the  south,  just  beyond  the 
border  of  the  State.  The  river  forms  a  third  of  the  western  boundary 
of  the  State,  receiving  along  this  portion  the  Owyhee,  Boise,  Payette, 
Weiser,  Salmon,  and  Clearwater  rivers,  all  draining  large  areas  of 
country.  Of  these,  the  Clearwater  rises  in  the  Bitter  Boot  Mountains; 
the  Salmon  drains  an  extensive  interior  area  between  the  Continental 
Divide  and  a  parallel  range,  the  Sawtooth,  in  the  middle  of  the  State; 
the  Boise  and  Payette  drain  from  the  latter  range  west;  the  Weiser 
occupies  a  longitudinal  valley  parallel  with  the  Snake,  from  which  it  is 
separated  by  a  narrow,  rugged  range,  the  continuation  of  the  Seven 
Devils  Mountains;  and  the  Owyhee  drains  the  southwestern  corner  of 
the  State.  The  Palouse  Eiver,  also  a  tributary  of  the  Snake,  drains  a 
small  scope  of  territory  just  north  of  the  Clearwater,  within  the  western 
edge  of  the  State. 

DRAINAGE  SYSTEM    OF    THE    COLUMBIA    RIVER. 

From  the  divide  north  of  the  Palouse  and  the  Clearwater  the  drain- 
age is  to  the  Columbia,  through  the  Spokane  Kiver,  Clarks  Fork,  and 
the  Kootenai,  the  first  of  these,  with  its  tributaries,  heading  within  the 
State,  in  the  Bitter  Root  Mountains,  the  others  crossing  it  from  Mon- 
tana. Of  the  larger  lakes  in  this  system,  Cceur  d'Alene  has  an  altitude 
above  sea-level  of  2,150  feet;  Pend  d'Oreille,  2,080  feet. 

The  distribution  and  relative  areas  of  the  several  drainage  basins 
are  shown  on  the  general  map. 


ELDEIDOE.]  DRAINAGE    AND    MOUNTAINS.  219 

MOUNTAINS. 

The  portion  of  Idaho  lying  north  of  the  Snake  Plains  is  an  intricate 
region  of  lofty  mountains  and  deep  canyons.  The  altitude  of  the  moun- 
tains varies  between  6,000  and  12,000  feet,  while  3,000  to  4,000  feet  is  a 
common  depth  for  the  canyons.  The  mountains  lie  either  "  en  masse  "- 
nearly  devoid  of  topographic  system — or  in  ranges.  The  development 
"en  masse"  is  due  to  early  geologic  accidents — fracturing,  faulting,  and 
folding — and  subsequent  modification  by  erosion,  the  region  being  one 
of  nearly  structureless  granite  of  almost  homogeneous  texture.  The 
range  form,  on  the  contrary,  is  the  effect  of  erosion  either  upon  pro- 
nounced folds  in  sedimentary  beds  and  foliated  granites  or  upon  a 
complex  in  which  there  existed  a  difference  in  the  texture  and  hardness 
of  the  rocks,  decay  and  disintegration  taking  place  more  rapidly  in 
one  than  in  another. 

The  range  of  first  importance  in  the  topography  of  Idaho  is  the 
Continental  Divide.  This  consists  of  folded  metamorphosed  beds — 
quartzites  and  schists — the  structural  and  topographic  axes  coincid- 
ing. Second  in  importance  are  the  ranges  dividing  the  chief  drainage 
basins.  Among  these  are  the  Sawtooth,  in  the  center  of  the  State, 
nearly  parallel  with  the  Continental  Divide;  two  north  and  south 
ranges  on  the  western  side  of  the  State — the  Seven  Devils  between  the 
Snake  and  Weiser  rivers,  and  a  shorter  ridge  between  the  latter  stream 
and  the  North  Fork  of  the  Payette;  a  transverse  ridge  of  irregular 
trend  extending  from  the  southern  end  of  the  main  Sawtooth  Range 
to  the  Continental  Divide,  and  separating  the  Salmon  waters  from 
those  of  the  Snake  to  the  south ;  a  ridge  north  of  and  having  the  same 
direction  as  the  last,  springing  from  the  Sawtooth  in  the  vicinity  of 
Cape  Horn,  and  constituting  the  watershed  of  the  main  and  middle 
forks  of  the  Salmon;  and  a  range  from  the  Sawtooth  west,  dividing  the 
Boise  and  Payette  drainage.  Of  these  secondary  ranges,  the  two  in 
the  western  part  of  the  State  are  unknown  to  the  writer ;  the  others 
were  crossed  once  or  twice.  The  Sawtooth  and  the  northern  of  the 
transverse  ridges  are  composed  chiefly  of  granite,  with  local  eruptives, 
while  the  southern  transverse  ridge  consists  in  part  of  granite  and  in 
part  of  more  or  less  metamorphosed  .sedimentary  beds  cut  by  eruptives 
and  folded.  Subordinate  still  to  the  foregoing  are  the  ranges  sepa- 
rating the  major  water  courses  within  the  several  drainage  basins. 
Particularly  noticeable  among  these  are  the  Salmon  and  Lost  River 
ranges  and  others  just  to  their  west,  in  eastern  Idaho,  largely  composed 
of  folded  metamorphic  rocks;  the  north  and  south  ranges  between 
the  several  forks  of  the  Salmon  River,  springing  from  the  northern  of 
the  transverse  ridges  and  consisting  chiefly  of  structureless  granite, 
except  at  the  east,  where  quartzites  prevail ;  the  ranges  between  the 
Boise  and  Payette  basins,  also  of  granite;  and  finally  the  Owyhee 
Range  in  southwestern  Idaho,  consisting  ot  granite  and  eruptives. 


220  GEOLOGICAL    RECONNAISSANCE   ACROSS    IDAHO. 

The  mountains  of  purest  range  type  occur  chiefly  in  the  eastern  part 
of  the  State,  in  proximity  to  the  Continental  Divide,  in  a  region  of 
uplifted  sedimentaries,  altered  or  unaltered  quartzites,  schists,  and 
limestones.  West  of  this  area,  in  the  region  of  granite,  the  ''en  masse" 
form  prevails;  but  the  range  type  is  still  present  in  the  Owyhee,  Boise, 
Trinity,  Sawtooth,  and  transverse  ranges,  where  the  granite  is  strongly 
foliated  or  the  rocks  are  of  different  texture  and  hardness  and  erosion 
has  removed  one  class  faster  than  another. 

CANYONS  AND  INTERMONTANE  VALLEYS. 

Throughout  the  mountain  region  are  many  quite  impassable  canyons, 
and  others  hardly  less  so.  The  early  prospectors,  prior  to  last  year 
(1894),  had  made  a  few  passable  with  trails,  and  recently  the  Salmon 
above  Salmon  City  has  been  opened  by  a  portion  of  the  State  wagon- 
road.  The  walls  of  the  canyons  are  precipitous,  sheer  drops  of  1,000 
feet  being  not  infrequent,  while  the  confining  slopes  rise  from  2,000  to 
3,000  feet  higher.  The  bottoms  are  filled  with  debris,  the  streams  being 
swift  and  wild.  Among  the  most  rugged  of  the  canyons  are :  the  canyon 
of  the  South  Fork  of  the  Boise ;  several  along  the  Salmon ;  that  of  the 
Middle  Fork  of  the  Salmon,  with  its  tributary,  Loon  Creek;  and  those 
of  the  numerous  branches  of  the  Clearwater. 

Within  the  mountains,  also,  are  occasionally  to  be  found  open  valleys 
from  10  to  20  miles  in  length  by  2  to  7  or  8  in  width.  They  have  become 
the  repositories  of  the  products  of  modern  erosion,  or  of  materials 
derived  from  the  inclosing  mountains  and  laid  down  in  Tertiary  times 
or  their  early  canyons  hav3  been  filled  to  depths  of  several  hundred 
feet  with  lava-flows,  to  be  recut  by  later  streams  to  their  present  levels. 

Prominent  among  these  valleys  are  the  following :  Along  the  portion 
of  the  Salmon  traversed,  one  at  the  head  of  the  main  fork,  east  of  the 
Sawtooth  Eauge.  There  is  here  a  stretch  of  nearly  40  miles  north  and 
south  by  C  or  8  east  and  west  occupied  by  material,  chiefly  crystalline 
and  eruptive,  derived  from  the  surrounding  mountains.  Long  and 
gentle  talus  slopes  border  the  valley,  while  its  center  is  a  broad  flat,  cut 
to  a  depth  of  5  to  30  feet  by  the  river  channel.  The  valley  was  prob- 
ably once  the  site  of  a  glacial  lake,-  heavy  beds  of  morainal  matter 
being  still  visible  along  the  base  of  the  Sawtooth. 

At  Challis  is  another  opening.  This  in  itself  is  10  or  15  miles  in 
diameter,  but  tributary  valleys  to  the  east  and  west  carry  the  general 
topographic  depression  some  10  to  15  miles  farther  back.  The  Salmon 
enters  the  valley  7  miles  southeast  of  Challis  through  a  sharp  canyon 
in  crystalline  and  eruptive  rocks,  and  after  a  northerly  course  of  about 
15  miles,  along  which  are  rich  bottom  and  bench  lands,  again  enters  the 
mountains  in  a  canyon  continuous — with  the  exception  of  a  small  open- 
ing in  the  vicinity  of  the  Pahsimeri — to  within  6  or  7  miles  of  Salmon 
City.  The  valley  of  Antelope  Creek,  the  tributary  from  the  east,  is 
broad  and  level,  the  stream  small.  The  valleys  to  the  west  are  more 


INTERMONTANE    VALLEYS.  221 

rolling,  but  the  creeks,  again,  carry  little  water.  There  is  a  deep  deposit 
of  Quaternary  gravel  in  the  Challis  Valley,  but  the  evidence  of  glacial 
action,  along  the  route  traversed  at  least,  is  not  nearly  so  pronounced 
as  in  the  upper  valley  of  the  Salmon  east  of  the  Sawtooth  Range. 

The  Pahsinieri  Valley  appears  open  for  fully  15  miles  from  its  mouth. 
It  is  6  or  7  miles  broad,  with  a  narrow  strip  of  bottom  land  along  the 
center,  the  remainder  being  bench  land  underlain  with  Quaternary 
debris  resting  upon  crystallines  and  eruptives. 

A  third  and  still  broader  iutermoutane  valley  of  the  Salmon  occurs 
about  Salmon  City.  This  is  an  illustration  of  the  mountain-locked 
valleys  that  have  become  repositories  of  materials  in  Tertiary  times. 
The  depth  of  the  pre-Tertiary  valley  is  unknown,  but  the  sediments 
indicate  it  to  have  been  several  hundred  feet  greater  than  at  present. 
The  extent  of  the  valley  is  about  20  miles  along  the  Salmon,  in  a  north 
and  south  direction,  by  10  miles  in  an  east  and  west  direction.  To  the 
southeast  it  is  continued,  in  the  Lemhi  Valley,  fully  30  miles,  maintain- 
ing here  a  width  of  5  to  8  miles.  A  long  spur  of  the  Salmon  River 
Range  lies  in  the  forks  of  the  valleys.  Both  the  Salmon  and  Lemhi 
valleys  present  gentle  slopes  from  side  to  center,  locally  rendered 
somewhat  uneven  by  the  folds  into  which  the  Tertiary  beds  have  been 
thrown.  Along  the  present  stream  channels,  particularly  the  Salmon, 
the  Tertiary  strata  frequently  form  precipitous  bluffs  50  to  80  feet 
high.  The  first-bottom  lauds  are  from  a  half  mile  to  1  or  2  miles  wide, 
but  there  is  generally  a  long,  evenly  sloping  bench  on  one  side  or  the 
other  which  is  susceptible  of  cultivation  wherever  water  is  available. 
The  configuration  of  the  valley  has  perhaps  changed  slightly  from  time 
to  time,  not  only  by  reason  of  erosion,  but  also  through  the  agency  of 
dynamic  movements,  indicated  by  flexures  in  the  Tertiary  strata. 

Of  the  other  intermontane  valleys,  that  of  Lost  River  was  traversed 
only  along  the  two  branches  forming  its  head.  The  northern  of  these 
has  its  source  in  the  Thousand  Springs,  near  Dickey.  The  valley  is 
here  3  or  4  miles  broad  and  very  level ;  just  below  it  opens  to  a  still 
greater  width  and  becomes  continuous  with  the  main  valley  below  the 
forks.  The  west  or  main  fork  has  a  narrow  bottom,  rarely  over  a  half 
mile  wide,  confined  between  rugged  hills  from  600  to  2,000  feet  high. 

The  valley  of  Wood  River  from  a  point  about  15  miles  above  Ketchum 
to  its  exit  from  the  mountains  maintains  a  width  of  bottom  of  between 
one-half  and  1J  miles.  The  floor  is  level,  and  is  underlain  with  a 
Quaternary  wash  of  varied  material.  The  tributary  valleys  are  usually 
sharp  mountain  gorges,  their  bottoms  rarely  over  one-quarter  mile  wide. 

From  Hailey  to  Boise  the  route  of  reconnaissance  lay  along  the 
southern  border  of  the  mountain  mass  of  Idaho ;  for  the  eastern  half 
of  the  distance,  in  the  broad,  prairie-like  valley  of  Camas  Creek ;  for 
the  western  half — from  High  Prairie  at  the  head  of  Camas  Creek  to 
the  Boise  Valley — within  the  foothills  of  the  mountains.  The  valley 
of  Camas  Creek  is  from  10  to  15  miles  wide,  extending  directly  from 


222  GEOLOGICAL   RECONNAISSANCE    ACROSS   IDAHO. 

the  base  of  the  mountains  on  its  north  to  the  low  range  of  hills — partly 
sedimentary,  partly  crystalline  or  eruptive — which  divides  it  from  the 
Snake  Plains  to  the  south.  The  channel  of  Camas  Creek  is  deep  cut,  but 
the  lateral  streams  are  rarely  depressed  more  than  10  or  15  feet.  Were 
the  water  supply  sufficient  the  valley  would  present  some  of  the  most 
favorable  agricultural  conditions  observed  during  the  reconnaissance. 

From  the  Camas  Valley  westward  for  a  distance  of  25  or  30  miles  is 
a  region  of  high,  irregularly  eroded  hills,  extending  from  the  mountains 
prairieward  10  to  15  miles.  The  drainage  of  this  area  is  partly  into  the 
Boise,  partly  direct  to  the  Snake  River.  The  divide  between  the  two 
waters  threads  its  way  with  irregular  trend  through  the  region,  passing 
finally  into  thegreat  lava  flat  which  constitutes  apart  of  the  Snake  Plains 
at  their  western  end.  From  this  divide,  at  the  head  of  Indian  Creek,  a 
high  range  passes  northwest  and  walls  the  Boise  intermontane  drainage 
basin  from  the  general  valley  without.  The  Boise  River  debouches  from 
a  rugged  defile  in  the  range  at  a  point  about  12  miles  southeast  of  Boise 
City.  In  the  vicinity  of  Little  Camas  Prairie,  however,  the  early  topo- 
graphic rim,  confining  on  the  south  the  intermontane  drainage  of  the 
South  Fork  of  the  Boise,  has  been  deeply  degraded,  and  but  for  the 
cutting  of  the  stream — its  recuttiug,  even,  through  the  comparatively 
recent  lavas — at  a  more  rapid  rate  than  the  denudation  of  the  rim,  the 
waters  of  the  South  Fork  would  have  found  an  exit  at  this  point  direct 
to  the  Snake.  In  the  hill  area  just  described  the  valley  of  greatest 
extent  is  that  of  the  Little  Camas,  7  or  8  miles  long  and  from  1  to  5 
miles  wide.  Meadow  lauds  occupy  its  upper,  narrower  portion,  while 
below  it  assumes  a  prairie  aspect  and  is  underlain  by  a  broad  flow  of 
lava.  A  ridge  2,000  feet  high  extends  across  the  southern  end  of  the 
valley,  and  continues  east  ward  for  several  miles  as  the  southern  border, 
also,  of  the  Big  Camas  Valley.  The  entire  foothill  region  between  the 
head  of  Big  Camas  Creek  and  the  main  Boise  Valley  west  of  the  moun- 
tains is  one  of  granite,  lava,  and  other  erupttves  in  dike  form,  and  the 
effect  of  such  varied  rock  assemblage  has  been  n  most  irregular  devel- 
opment of  topographic  features.  The  lava  generally  occurs  as  a  thin 
sheet  or  a  succession  of  thin  sheets  upon  the  granite,  and  wherever  it 
is  present  the  country  becomes  rough  and  difficult  to  travel. 

North  of  the  region  just  described' and  within  the  rim  of  the  Boise 
drainage  basin  is  a  flat,  8  to  10  miles  in  diameter,  known  as  Smiths 
Prairie.  The  floor  is  wholly  of  lava,  which  extends,  also,  for  some  dis- 
tance into  the  tributary  valleys.  The  surface  is  locally  smooth,  rugose, 
or  gently  undulating,  and  in  only  one  or  two  instances  is  it  relieved  by 
low  hills.  The  general  altitude  of  the  surface  is  about  4,800  feet,  a 
little  lower  than  the  small  lava  prairie  a  few  miles  to  the  southeast  at 
the  mouth  of  Little  Camas  Creek.  Along  the  southern  edge  of  the 
prairie  the  South  Fork  of  the  Boise  has  cut  a  gorge  over  1,000  feet  deep, 
in  the  precipitous  walls  of  which  appear  two  seemingly  distinct  lava- 
flows,  one  occupying  the  upper  500  to  700  feet  of  the  gorge,  the  other  200 
to  300  feet  at  the  bottom.  The  relative  ages  of  these  flows  were  not 


ELDRIDOE.;  GLACIAL   ACTION.  223 

determined,  but  either  might  be  the  older.  The  flows  may  be  traced 
at  intervals  the  entire  length  of  the  South  Fork,  and  also  along  the 
main  stream  below,  to  its  debouchment  from  the  mountains.  At  the 
mouth  of  Moores  Creek  they  are  joined  by  others  which  extend  nearly 
to  Idaho  City.  Usually  the  lava  forms  narrow  ribbons  adhering  to 
the  sides  of  the  canyons.  These  appear  to  drop  in  altitude  as  the  can- 
yon is  descended,  indicating  an  ancient  lava  river  down  an  earlier- 
eroded  gorge,  with  occasional  lava  lakes  in  the  openings. 

Twenty-eight  miles  north  of  Boise,  at  Horseshoe  Bend,  on  the  Pay- 
ette,  is  a  small  mountain  locked  valley  carrying  deposits  of  Tertiary 
age.  The  length  of  the  valley  is  10  to  12  miles,  the  width  4  or  5  miles, 
the  trend  northeast.  Along  the  western  edge  the  Payette  flows  for  a 
distance  of  5  or  6  miles,  entering  and  leaving  by  a  sharp,  rugged  canyon 
in  the  granite  of  the  Boise  Kauge.  The  valley  is  apparently  one  of 
erosion.  Its  configuration  is  somewhat  irregular,  both  upon  original 
outlines  and  from  the  later  encroachment  of  the  heavy  talus  slopes 
along  the  base  of  the  mountains.  Moreover,  the  underlying  Tertiary 
beds  here  and  there  project  through  the  Quaternary  in  buttes  or  ridges. 
Gentle  folding  has  taken  place  here,  as  in  the  valley  at  Salmon  City. 

One  of  the  most  peculiar  valleys  encountered,  of  which  the  topo- 
graphic origin  was  not  determined,  is  that  locally  known  as  "  Prairie 
Basin,"  about  45  miles  southwest  of  Leesburg.  This  is  a  high  inter- 
montane  depression,  apparently  of  considerable  depth  originally,  but 
since  filled  by  heavy  deposits  of  glacial  drift,  which  in  still  later  times 
has  been  partially  removed  through  the  channels  of  Big  Creek  and  its 
tributaries.  The  valley  surface  is  now  in  long,  rounded  ridges  and 
hillocks.  Owing  to  its  altitude,  to  the  extent  of  opening,  and  to  the 
heavy  growth  of  grass  covering  its  every  acre,  the  "basin"  is  easily 
recognized  from  distant  peaks. 

GLACIAL  ACTION. 

Evidences  of  early  glaciers  were  observed  at  many  points  along  the 
route  of  reconnaissance,  and  it  is  probable  that  their  existence  was  quite 
general  throughout  the  mountain  region  of  Idaho.  Conspicuous  local- 
ities in  the  southern  half  of  the  State  are  the  Trinity  and  Sawtooth 
ranges.  In  the  upper  portions  of  many  of  the  valleys  heading  in  the 
former  are  glacial  bowlders,  grooved  rock  surfaces,  roches  moutonnees, 
and  lateral  and  terminal  moraines.  North  of  the  Trinity  Eauge,  and 
also  east  of  the  Sawtooth,  are  numerous  glacial  lakes,  1  to  2  miles  in 
diameter,  their  waters  held  back  by  terminal  moraines.  East  of  the 
Sawtooth  the  mass  of  debris  from  the  earlier  glaciers  is  enormous, 
extending  50  or  60  miles  along  its  front  and  forming  a  belt  from  5  to  8 
miles  broad.  This  is  cut  by  the  streams  of  the  present  day,  and  now 
forms  a  rough,  often  heavily  timbered  slope  most  difficult  to  traverse. 
Other  observed  localities  are  the  ranges  west  of  Salmon  City,  particu- 
larly about  the  head  of  Panther  Creek  and  the  several  tributaries  of 
the  Yellow  Jacket,  a  branch  of  Carnas  Creek.  Prairie  Basin  has  already 


224  GEOLOGICAL   RECONNAISSANCE   ACROSS   IDAHO. 

been  noted  as  the  possible  site  of  an  ancient  glacial  lake,  the  heavy 
deposit  of  bowlders  having  been  derived  from  the  surrounding  moun- 
tains. Indeed,  nearly  all  of  the  valleys  traversed  in  the  reconnaissance 
show,  at  their  heads,  more  or  less  evidence  of  former  glaciers.  Even 
now  ice  action  is  rife  at  the  higher  altitudes,  where  the  snows  are  exceed- 
ingly deep  and  their  disappearance  is  rarely  complete  from  one  season 
to  another. 

THE  FORMATIONS. 

The  rocks  occurring  in  the  southern  half  of  Idaho  embrace  granites, 
gneisses,  syenites,  schists,  quartzites,  limestones,  calcareous  and  non- 
calcareous  shales,  sandstones,  clays,  and  eruptives  in  great  variety. 
The  granites  and  syenites,  in  part  at  least,  are  probably  of  the  Archeau 
age;  the  schists,  Algonkian.  The  quartzites  are  distributed  from  Algou- 
kian  to  Carboniferous,  while  the  limestones  may  include  both  Silurian 
and  Carboniferous.  The  age  of  the  great  calcareous  shale  series  of  the 
Wood  River  and  neighboring  districts  is  undetermined,  but  the  evidence 
points  to  the  Carboniferous.  It  is  undoubtedly  Paleozoic.  The  sand- 
stones and  clays  encountered  are  all  of  Tertiary  age,  Eocene  ( ?)  and 
Neocene.  Post- Pliocene  gravels  are  abundant.  The  eruptives  are  possi- 
bly of  all  ages,  from  Archean  to  Recent.  The  assignment  of  the  several 
series  of  rocks,  except  the  sub-Carboniferous  and  Tertiaries,  is  provi- 
sional, being  without  fossil  evidence;  lithology  and  stratigraphy  alone 
form  the  basis  of  reference.  Fossils  might  be  found  at  several  horizons, 
but  the  exigencies  of  the  trip  did  not  permit  careful  search. 

GRANITES  AND  METAMORPHIC  ROCKS. 
ARCHEAN. 

To  this  group  are  provisionally  assigned  the  granite  and  gneiss,  but 
there  are  instances  where,  by  reason  of  included  bands  of  calcareo- 
micaceous  or  quartzitic  slates,  this  reference  to  the  Archean  instead  of 
the  Algonkiau  is  questionable.  Again,  it  is  probable  that,  in  part  at 
least,  the  granite  is  of  igneous  origin. 

The  granite  is  of  wide  occurrence,  and,  in  the  main,  of  a  single  type, 
with  three  or  four  regional  modifications.  The  type  rock  is  gray,  mod- 
erately coarse  in  texture,  and  composed  of  feldspar,  quartz,  and  mica, 
with  few  accessory  minerals.  The  feldspar  is  white,  chiefly  orthoclase, 
with  perhaps  an  occasional  small  amount  of  plagioclase.  The  orthoclase 
is  often  porphyritically  developed,  while  the  color  varies  locally,  but 
rarely,  to  a  faint  pink.  The  quartz  is  generally  granular.  The  mica 
includes  both  the  black  and  white  varieties,  the  former  predominating, 
but  showing  considerable  variation  in  amount.  It  is  distributed  irreg- 
ularly throughout  the  mass  of  the  rock,  or  is  lineally  disposed,  when  it 
imparts  a  more  or  less  definite  foliation.  An  accessory  mineral  is  horn- 
blende, in  fine  and  coarse  crystals,  but  on  the  line  of  reconnaissance  its 
presence  is  local  and  somewhat  rare.  It  occurred  notably  in  the  granite 


ELDB.DOK.]  THE   FORMATIONS.  225 

of  Trinity  Range,  8  or  9  miles  west  of  Rocky  Bar,  and  again  in  con- 
spicuously large  crystals  along  the  lower  portion  of  Napias  Creek,  20 
to  30  miles  west  of  Salmon  City.  In  the  latter  locality  a  further  modi- 
fication of  the  type  granite  takes  place  in  the  development  of  the 
orthoclase  crystals  to  an  extraordinary  size — from  1  to  3  or  4  inches  in 
the  direction  of  the  vertical  axis.  The  outlines  of  the  crystals  are 
usually  somewhat  rounded.  On  exposed  rock  surfaces  the  crystals 
weather  in  knobs,  but  fresh  fractures  extend  through  rock  and  crystals 
alike,  often  along  cleavage  planes  of  the  latter,  the  rock  then  present- 
ing the  appearance  of  a  more  or  less  uniformly  crystalline  granular 
matrix  sharply  relieved  by  the  smooth,  brilliant  faces  of  the  porphy- 
ritically  developed  feldspar.  To  the  granite  of  this  description  the 
designation  "bird's-eye"  may  be  appropriately  applied. 

A  second  departure  from  the  type  granite  occurs  in  the  precipitous 
walls  of  the  Loon  Creek  Canyon,  3  or  4  miles  below  Oro  Grande. 
Here  the  gray  variety  is  wholly  replaced  by  one  of  deep  pink,  derived 
from  orthoclase  of  this  color.  A  faint  greenish  tint  is  sometimes 
induced  by  the  decomposition  of  the  biotite.  The  texture  of  this 
granite  is  fine  to  coarse,  and  the  grain  even.  The  areal  extent  is  unde- 
termined, but  it  outcrops  at  intervals  for  several  miles. 

A  third  and  local  modification  of  the  normal  granite  takes  place  in 
proximity  to  mineral  veins.  This  consists  in  the  loss  of  a  very  large 
proportion  of  the  mica,  the  remaining  feldspar  and  quartz  constituting 
a  somewhat  conspicuous  rock,  by  means  of  which  the  course  of  the  vein 
may  readily  be  traced,  and  which,  in  fact,  may  enter  into  the  composi- 
tion of  the  vein  itself  as  "ledge  matter."  The  clearest  illustration  of 
this  occurrence  is  to  be  seen  in  the  mining  camps  in  the  vicinity  of 
Rocky  Bar  and  Atlanta. 

Typical  gneiss  occurs  in  some  of  the  spurs  of  the  Sawtooth  Range, 
notably  about  the  drainage  system  of  Upper  Redfish  Lake.  Wherever 
observed  it  has  the  mineral  composition  of  the  typical  gray  granite. 
In  the  locality  referred  to,  the  foliation  is  so  complete  as  to  create  the 
impression  of  bedded  strata,  while  the  effect  upon  topographic  lines  is 
markedly  that  of  an  unaltered  stratified  rock. 

The  granites  and  gneisses  prevail  in  the  mountains  of  the  western 
half  of  the  State,  while  in  the  eastern  they  occur  as  range  cores  in  con- 
nection with  schists,  quartzites,  or  limestones.  None  of  the  latter 
rocks,  however,  were  encountered  in  the  western  half  of  the  State. 

ALGONKIAN. 

To  this  is  provisionally  assigned  the  great  series  of  micaceous,  quartz- 
itic,  and  chloritic  schists  of  eastern  Idaho.  The  reference  is  based 
merely  upon  lithological  character  and  a  resemblance  to  other  beds  in 
the  Cordilleras  which  have  already  been  so  assigned.  The  series 
embraces,  together  with  the  schists,  numerous  beds  of  quartzite,  and  all 
have  the  general  field  appearance  of  clastic  rocks.  Many  of  the  layers 
16  GEOL,  PT  2 15 


226  GEOLOGICAL   RECONNAISSANCE    ACROSS   IDAHO. 

contain  a  very  considerable  amount  of  carbonate  of  lime.  The  series  in 
regions  of  strong  development  has  a  probable  thickness  of  3,000  to 
4,000  feet,  and  is  believed  to  be  unconlbrmable  with  the  granite.  In 
any  event  there  was  a  time-break  prior  to  the  deposition  upon  the 
granite  of  the  superincumbent  strata,  for  the  same  series  does  not  every- 
where follow  in  the  areas  brought  under  observation. 

The  region  of  crystalline  schists  is  distinctively  the  eastern  half  of 
the  State,  although  in  this  portion  are  found  also  the  Paleozoic  meas- 
ures, as  well  as  the  older  granites  forming  cores  of  many  of  the  ranges. 
The  strongest  development  of  the  series  observed  on  the  route  of  recon- 
naissance was  in  the  Continental  Divide  east  of  Salmon  City,  in  the 
region  of  Big  Creek  and  its  tributaries,  and  in  the  range  separating 
the  drainage  of  this  stream  from  that  of  the  main  Salmon.  The  series 
is  also  exposed  at  several  points  along  the  Salmon  above  Salmon  City> 
notably,  from  a  point  about  9  miles  above  the  city  to  one  18  miles  above. 
It  is  then  cut  out  by  eruptives  of  undetermined  extent ;  after  a  partially 
covered  stretch  of  10  miles  it  again  appears  along  the  river  for  a  dis- 
tance of  about  2i  miles;  eruptives  again  succeed,  the  schists  finally 
disappearing  in  a  small,  disconnected  outcrop  near  the  mouth  of  the 
Pahsimeri. 

Schistose  rocks  occur  associated  with  the  slates  of  Lost  and  Wood 
rivers,  but  it  is  somewhat  doubtful  if  they  belong,  in  their  entirety  at 
least,  to  the  great  series  of  crystalline  strata  just  described.  They  are 
here  apparently  associated  more  closely  with  Paleozoic  measures. 

UNALTERED    SEDIMENTARY   ROCKS. 
PALEOZOIC. 

The  succession  of  beds  in  southern  Idaho  is  difficult  of  determina- 
tion, owing,  first,  to  a  marked  difference  in  their  petrographic  charac- 
ters from  recognized  formations  elsewhere  in  the  Rocky  Mountains, 
this  preventing  their  assignment  to  a  definite  position  in  the  scale  of 
formations;  secondly,  to  frequent  interruptions  of  continuity  by  erup- 
tives; and  in  the  third  place,  to  folds  and  faults. 

Later  than  the  granites,  and  probably  also  than  the  schists  described 
above,  is  a  great  body  of  pink  and  white  quartzites,  of  at  least  1,500 
feet  maximum  thickness.  They  are  heavy  bedded,  hard,  and  uniform 
in  texture  and  composition.  Their  greatest  development  is  along  the 
Salmon  River  from  a  point  about  5  or  6  miles  below  the  mouth  of  the 
Pahsimeri  to  within  6  or  7  miles  of  Challis.  At  the  lower  end  of  this 
stretch  they  are  thrown  into  a  prominent  anticline,  the  eastern  end  of 
which  is  cut  by  the  sharp  gorge  of  the  river,  showing  them  in  uninter- 
rupted succession  for  1,000  to  1,500  feet.  Numerous  other  similarly 
disposed  anticlines  occur  between  this  point  and  Challis,  all  in  these 
measures.  In  none  was  Archean  observed,  and  none  was  capped  by 
higher  strata  of  different  nature.  A  small  exposure  of  mica  and  quartz- 
itic  schists  (Algonkian?)  occurs  just  below  the  mouth  of  the  Pahsimeri, 


ELDRIDOE]  PALEOZOIC    MEASURES.  227 

and  although  of  the  same  strike  as  the  quartzites  on  either  side,  they 
are  separated  from  them  by  intervals  of  3  miles  or  over  occupied  by 
eruptives  or  a  Quaternary  wash,  and  the  relations  between  the  two 
series  are  thus  obscured.  On  Deer  Creek,  about  6  miles  above  its  con- 
fluence with  Wo»d  River,  is  a  local  body  of  quartzite,  which  somewhat 
resembles  those  just  described  along  the  Salmon.  It  rests  directly 
upon  Archean  granite,  and  is  overlain  by  the  dark-blue  and  black 
calcareous  slates  which  form  so  import;uit  a  feature  of  the  Wood  River 
district.  The  areal  extent  of  this  quartzite  was  not  determined,  but 
it  is  apparently  small.  A  little  south  of  Deer  Creek  it  disappears 
with  marked  rapidity,  the  slates  coming  down  on  the  granite.  Similar 
quartzites  probably  exist  in  large  bodies  in  the  Salmon  River  Range 
also. 

This  quartzite  is  evidently  to  be  classed  with  the  older  sedimentary 
rocks  encountered;  moreover,  it  closely  resembles  the  established  Cam- 
brian quartzite  of  Colorado;  on  these  grounds  it  is  here  referred  to  the 
Cambrian. 

In  the  high  range  of  mountains  forming  to  the  south  the  watershed 
of  Wood  River  and  to  the  north  and  northeast  that  of  Salmon  and  Lost 
rivers  are  several  thousand  feet  of  quartzites,  slates,  conglomerates, 
calcareous  shales,  and  limestones,  which  it  has  been  impossible  in  the 
time  available  either  to  segregate  into  formations  or  to  refer  to  definite 
ages.  The  range  itself  is  the  southeast  extension  of  the  Sawtooth,  and 
carries  some  of  the  loftiest  peaks  in  Idaho,  notable  among  them  being 
Mount  Hyndman,  12,000  feet,  10  to  15  miles  southeast  of  Ketchum. 
The  range  is  an  anticline,  the  core  being  gray  granite,  of  the  same  type 
as  that  of  the  main  Sawtooth,  Trinity,  and  Boise  ranges,  and  with  a 
width  of  exposure  along  the  Ketchum-Challis  ro:id  of  between  2  and  3 
miles.  Resting  upon  the  granite  on  both  the  north  and  south  sides  of 
the  range  is  the  series  of  sedimentary  beds  mentioned  above.  The 
stratigraphic  succession  of  the  several  members  of  the  series  is  not 
confidently  determined,  and,  moreover,  there  is  an  apparent  difference 
in  the  succession  on  the  two  sides  of  the  range. 

On  the  north  side,  about  the  heads  of  the  several  branches  of  Lost 
River,  quartzites  and  slates,  dipping  northeasterly,  follow  one  another 
in  quick  succession.  Black  limestones  are  also  present,  and  in  some 
localities,  not  visited,  they  are  apparently  of  considerable  importance, 
judging  from,  bowlders  encountered  in  the  valley.  It  is  doubtful,  how- 
ever, if  any  single  layer  is  more  than  a  few  feet  thick.  For  2  miles  above 
East  Fork  of  Lost  River,  large  bodies  of  lava  occupy  both  valley  and 
hillsides,  interrupting  the  series  of  sedimentary  beds.  Below  Bast  Fork, 
however,  the  quartzites  and  limestones,  or  at  least  quartzites  with 
strongly  calcareous  layers  included,  reappear,  continuing  for  8  or  9 
miles  down  the  valley  to  the  point  where  the  road  turns  from  the  river 
northward  toward  Dickey.  Within  this  distance  the  quartzites  per- 
haps predominate,  but  black  and  probably  often  calcareous  slates  are 


228  GEOLOGICAL    RECONNAISSANCE   ACROSS    IDAHO. 

not  infrequent.  The  calcareous  nature  of  some  of  the  quartzite  layers 
is  particularly  marked  a  short  distance  below  East  Fork.  For  the 
lower  2  miles  of  the  distance  there  occur  in  the  series  a  number  of  con- 
glomerate layers  1  to  10  feet  thick.  These  are  composed  of  a  mass  of 
cherty,  subangular  pebbles,  and  in  the  heavier  beds,  of  quartzite  and 
limestone  debris  in  addition,  the  whole  cemented  by  flue,  granular 
material  of  the  same  nature.  East  of  the  Dickey  road,  after  its  turn 
northward  from  the  valley  of  Lost  River,  what  are  probably  sub-Car- 
boniferous limestones  appear  in  a  prominent  hill,  a  spur  of  the  high 
range  to  the  south.  This  limestone,  which  overlies  the  quartzite  and 
slate  series,  outcrops  in  great  bodies  in  the  mountains  to  the  north, 
forming  the  western  periphery  of  the  Thousand  Springs  Valley. 

South  of  the  Sawtooth  Range,  along  Trail  Creek  and  the  Ketch  urn 
road,  the  succession  of  beds  is  somewhat  different  from  that  on  the 
north  side.  Dark  quartzites  prevail  immediately  above  the  granite, 
followed  after  several  hundred  feet  by  a  thick  zone  of  light-colored, 
white  and  gray  quartzites.  About  6  miles  from  the  summit  the  entire 
series  is  repeated,  though  whether  in  natural  succession  or  by  faulting 
was  not  ascertained.  It  is  then  overlain  by  several  hundred  feet  of 
dark-gray  and  black  slates  or  shales,  probably  often  calcareous,  which 
continue  nearly  to  the  mouth  of  Trail  Creek  Canyon.  At  this  point 
the  succession  is  interrupted  by  eruptives,  which  extend  for  a  consider- 
able distance  along  the  Wood  River  Valley  on  its  eastern  side.  West 
of  Wood  River,  opposite  Ketchum,  is  a  heavy  body  of  limestone,  some- 
what resembling  in  its  massive  character,  its  mode  of  weathering,  and 
its  general  appea ranee  the  sub-Carboniferous  of  the  Rocky  Mountains, 
but  the  age  was  not  definitely  determined.  This  limestone  is  called  by 
the  miners  the  "gray  limestone,"  in  distinction  from  the  "blue  lime- 
stone," which  occurs  as  thin  beds  in  the  series  of  shale  overlying.  The 
dip  is  here  down  the  river  (southward).  The  overlying  shales  are  gray, 
dark-blue,  and  black,  and  carry  numerous  one  foot  to  six-foot  lime- 
stone bauds  distributed  through  them.  The  thickness  of  this  shaly 
series  is  estimated  at  5,000  feet.  It  constitutes  the  principal  ore- 
bearing  series  of  the  Wood  River  district,  though  the  lower  slates  and 
quartzites  occasionally  carry  mineral. 

On  Deer  Creek,  a  tributary  of  Wood  River  from  the  west,  an  erup- 
tive again  cuts  into  the  sedimentary  beds  and  occupies  the  hills  to  the 
north  of  the  valley  for  a  mile  above  its  mouth.  Above  this,  however, 
quartzites  come  in,  followed  by  a  few  hundred  feet  of  heavy-bedded 
dark-blue  limestones,  which  bear  considerable  resemblance,  in  texture, 
weathering,  and  general  appearance,  to  the  sub-Carboniferous.  Black 
calcareous  slates  of  the  general  character  of  those  already  described 
follow  the  limestones,  and  are  in  turn  succeeded  by  granite,  just  above 
Warm  Springs,  4  miles  above  the  mouth  of  the  creek.  This  succession 
of  shales  by  granite  is  possibly  due  to  faulting.  The  granite  extends 
up  the  valley  for  about  3  miles,  when  400  or  500  feet  of  the  heavy-bedded 


ELDRIDOE.I  PALEOZOIC    MEASURES.  229 

Cambrian-like  quartzite  succeed,  overlain  by  blue  and  black  slates  and 
limestones  similar  to  those  noted  farther  down  the  creek.  The  quartz 
ites  of  this  second  area  of  sedimentary  beds  did  not  appear  in  the 
first  series,  east  of  the  granite  at  the  Warm  Springs.  The  dip  of 
the  series  below  the  Warm  Springs  is  doubtful ;  west  of  the  granite 
block,  however,  it  is  westward,  bending  around  gradually  to  the  south- 
west and  south  at  the  head  of  Deer  Creek  and  its  tributary  from  the 
south,  Bed  Cloud  Gulch.  The  black  shales  and  limestones  of  Deer 
Creek  are  undoubtedly  a  part  of  the  general  series  which  constitutes 
the  leading  formation  of  the  Wood  River  district,  but  their  horizon  in 
the  latter  is  undetermined.  South  of  Deer  Creek,  east  of  Red  Cloud 
and  Narrow  Gauge  gulches,  the  blue  calcareous  slates  are  in  direct  con- 
tact with  the  granites  to  the  east,  the  quartzite  observed  in  the  valley 
of  Deer  Creek  having  here  disappeared.  Whether  faulting  or  iion- 
deposition  is  the  cause  of  irregularity  in  this  succession  of  the  beds  is 
unknown. 

It  is  possible  that  the  foregoing  series  of  rocks  from  the  granite  up, 
perhaps  10,000  to  15,000  feet  in  all,  fall  wholly  within  the  Paleozoic- 
Cambrian  and  younger  systems;  but  except  in  the  case  of  the  sub-Car- 
boniferous and  overlying  shales,  neither  their  age,  division  lines,  nor 
intersuccession  was  satisfactorily  determined.  The  heavy  bed  of  white 
and  pink  quartzite  on  Deer  Creek  belongs,  perhaps,  to  the  Cambrian; 
the  dark  quartzite  series  at  the  head  of  Trail  Creek,  and  the  calcareous 
slates,  quartzites,  and  conglomerates  of  Lost  River — from  an  unknown 
member  of  which  the  one  or  two  fossils  collected  have  been  determined  by 
Mr.  C.  D.  Walcott  to  be  not  older  than  the  Trenton — are  apparently  of 
a  post-Cambrian  age;  the  sub-Carboniferous  is  recognized ;  but  the  over- 
lying shales  are,  again,  beyond  a  general  reference  to  the  Carboniferous, 
in  doubt.  Lithologically  they  bear  a  slight  resemblance  to  the  Weber 
of  Colorado  and  Utah.  In  the  entire  Wood  Eiver  district  there  is 
much  and  varied  faulting  as  well  as  folding,  and  only  work  in  great 
detail  will  effect  a  solution  of  the  many  geological  problems  presented. 

The  foregoing  series  doubtless  occurs  in  many  ranges  to  the  east  and 
northeast  of  the  localities  described,  especially  in  the  mountains  east 
of  the  Thousand  Springs  Valley  and  of  Lost  River. 

The  sub-Carboniferous,  which,  of  the  Paleozoic  series  is  most  clearly 
recognized,  is,  excepting  possibly  in  the  Wood  River  region,  repre- 
sented by  the  massive  blue  cave  limestone,  so  characteristic  of  it 
throughout  the  Rocky  Mountain  region.  Its  identification  is  based 
upon  lithological  resemblances  and  the  contained  fossils,  although  no 
collection  of  fossils  was  attempted.  The  thickness  is  between  100  and 
400  feet.  The  limestone  occurs  in  especial  force  in  the  several  ranges 
between  the  Challis  Valley  and  the  main  fork  of  Lost  River,  but 
it  is  said  to  be  present  in  many  localities  in  eastern  Idaho.  Its  rela- 
tions to  underlying  beds  were  unobserved  except  in  a  single  locality, 
in  the  forks  of  Lost  Eiver,  where  it  apparently  succeeds  the  series  of 


230  GEOLOGICAL    RECONNAISSANCE   ACROSS    IDAHO. 

calcareous  quartzites,  slates,  and  conglomerates  described  above.  On 
the  divide  between  Antelope  Creek  and  Lost  River  it  is  overlain  by 
drab  calcareous  shales  resembling  those  in  the  Wood  River  district, 
though  possibly  less  metamorphosed. 

In  the  range  of  mountains  forming  the  southwest  side  of  the  Warm 
Spring  and  Antelope  valleys,  just  east  of  the  Salmon  River,  are  beds, 
possibly  500  to  600  feet  in  total  thickness,  which  are  partly  quartzite, 
partly  limestone,  the  latter  pinkish-drab,  massive,  and  somewhat 
resembling  the  Silurian  limestones  of  the  Colorado  areas.  In  the  hur- 
ried examination  given  them,  however,  no  fossils  were  found.  In  this 
locality,  also,  on  the  lower  flanks  of  the  range,  are  numerous  large 
hot-spring  deposits,  the  springs  being  now  extinct.  At  the  eastern 
end  of  the  range,  overlying  what  from  a  distance  is  apparently  the 
sub- Carboniferous  limestone,  are  about  50  feet  of  red  beds  of  unknown 
composition.  Above  these  are  several  hundred  feet  of  calcareous 
shales,  the  same  as  those  already  mentioned  on  the  divide  between 
Antelope  Creek  and  Lost  River.  The  Little  Lost  River  Range  east  of 
the  Antelope  and  Thousand  Springs  valleys  is  reported  by  prospectors 
as  being  composed  of  the  sub-Carboniferous  limestone  and  overlying 
shales,  and  from  a  distance  this  seems  to  be  the  case. 

CENOZOIC. 

This  system  is  represented  in  Idaho  by  the  great  series  of  sedimen- 
tary beds  underlying  the  Snake  Valley  and  by  others  which  occupy 
certain  of  the  intermontane  valleys.  Paleobotanic  evidence  points  to 
the  Eocene  or  Miocene  as  the  age  of  the  intermontane  sediments,  and 
molluscan  and  mammalian  remains  to  the  Pliocene  as  that  of  the 
Snake  River  beds. 

The  materials  of  the  Snake  River  beds  were  derived  largely  from  the 
mountains  to  the  north  and  south  of  the  valley,  though  do-ibtless  more 
or  less  detritus  from  the  region  of  the  headwaters  has  been  commingled 
with  the  other  sediments  for  its  entire  length.  The  material,  in  the 
western  half  of  the  State  at  least,  is  chiefly  of  granitic  origin,  consist- 
ing of  quartz  and  feldspar  grains,  often  with  a  kaolin-looking  cement, 
slightly  ferruginous.  The  prevailing  rock  is  a  flue  to  medium  grained, 
friable,  gray  sandstone,  but  clays  occur,  and  also  conglomerates.  The 
formation  was  examined  only  locally,  and  there  are  doubtless  many 
variations. 

Associated  with  the  Snake  River  beds,  and  in  some  instances  inter- 
stratifled  with  them,  are  flows  of  the  lava  which  in  one  locality  or 
another  is  such  a  well-known  feature  of  the  Snake  Plains.  Apparently 
the  outpouring  of  this  rock  took  place,  in  part  at  least,  while  the  sedi- 
mentary beds  were  still  being  deposited.  The  sediinentaries  outcrop 
along  the  Snake  River  in  bluffs  from  one  hundred  to  several  hundred 
feet  high,  and  also  occur  in  bent-lies  of  considerable  elevation  next  to 
the  mountains  both  north  and  south. 


ELDRIDGE.)  CENOZOIC    IN    SALMON-LEMHI    VALLEY.  231 

The  age  of  the  Snake  River  beds  is  probably  Pliocene.  Melania  tay- 
lori  Gabb  and  Liihasia  antiqua  Gabb,  together  with  an  undetermined 
vertebra  (carniverous),  have  been  found  by  Mr.  Arthur  Foote,  at 
Glenns  Ferry,  on  the  Snake,  in  Elmore  County,  and  Prof.  O.  C.  Marsh 
personally  reports  Pliocene  mammalia  from  the  same  series  of  beds  on 
the  north  side  of  the  Boise  River,  a  few  miles  below  Boise.1 

The  intermontane  valley  of  the  Salmon  and  Lemhi  rivers,  in  the 
center  of  which  Salmon  City  is  situated,  is  occupied  by  a  considerable 
thickness  of  Tertiary  beds,  the  materials  for  which  were  derived  from 
the  early  formations  of  the  inclosing  ranges.  These  materials  are,  for 
the  most  part,  of  granitic  or  quartzitic  debris,  and  the  beds  are  either 
clays,  sandstones,  or  conglomerates. 

The  clays  occupy  a  large  area  in  the  center  of  the  valley.  They  are 
light-green  and  red,  forming  a  conspicuous  feature  in  the  landscape. 
The  colors  are  in  two  zones,  the  red  at  the  higher  level,  nearer  the 
periphery  of  the  valley,  and  possibly  a  coloration  of  later  times. 
The  clays  are  slightly  arenaceous  in  some  layers,  in  others  distinctly 
sandy,  carrying  even  occasional  thin  bands  of  sandstone  or  conglom- 
erate near  the  top.  This  conglomerate  is  a  mass  of  small,  round,  or 
lenticular  pebbles  of  half-inch  maximum  diameter,  chiefly  quartzite. 
Near  the  same  horizon  are  thin  layers  of  hard,  white  or  brown,  homo- 
geneous clay,  which  usually  has  an  abundance  of  minutely  divided 
vegetable  matter  through  it,  sometimes  sufficient  to  render  it  lignitic. 
Here  and  there  fragmentary  stumps  of  trees  are  found  in  a  carbosilici- 
fied  condition. 

The  sandstones  of  this  intermontane  Tertiary  series  occur  at  the 
base  of  the  measures  exposed,  just  beneath  the  clay  division,  and  in 
all  are  probably  400  or  500  feet  thick.  They  outcrop  in  bold  bluffs 
along  the  Salmon.  Their  material  is  chiefly  quartz.  They  are  light 
yellow  or  gray  and  for  the  most  part  massive,  though  in  some  layers 
thin  bedded  and  even  shaly.  The  gray  beds  are  usually  the  finer  in 
texture,  the  yellow  sometimes  approaching  a  grit.  The  series  carries 
plant  remains,  leaves,  stems,  etc.,  which  locally  are  in  sufficient 
quantity  to  form  with  the  shale  a  lignitic  band.  In  one  instance,  just 
below  Salmon  City,  this  carbonaceous  matter  is  distributed  through  4 
or  5  feet  of  shales,  forming  a  coaly  slate,  within  which  is  a  6-inch  bed 
of  dark-brown,  woody  lignite.  So  far  as  at  present  exposed,  this  is 
not  of  economic  value.  Overlying  the  slate  are  10  feet  of  sandstone, 
succeeded  by  5  or  10  feet  more  of  a  very  hard,  moderately  coarse 
quartzite-conglomerate,  which  is  somewhat  ferruginous,  and  often 
fractures  across  matrix  and  pebbles  alike.  Within  a  short  distance  of 
this  outcrop  is  another  exposure  of  a  similar  conglomerate,  in  appear- 
ance 30  or  40  feet  higher  up  than  the  first,  but  not  definitely  so  deter- 
mined. This  second  conglomerate  in  particular  bears  a  resemblance 


'See  also  various  references  in  Bull.  U.  S.  Geol.  Survey  No.  84,  Correlation  papers— Neocene,  by 
Ball  and  Harris,  pp.  285  and  286, 1892. 


232      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

to  certain  beds  along  the  base  of  the  Continental  Divide  in  the  vicinity 
of  the  Lemhi  placer  mine  on  Kirtley  Creek.  Just  beneath  this  con- 
glomerate bed,  along  the  Salmon,  are  some  white  sandstones,  more  or 
less  argillaceous  and  ferruginous,  carrying  kidney  shaped  iron  concre- 
tions, and  leaf-bearing.  Among  the  forms  collected  Mr.  Kuowltou  has 
recognized — 

Sequoia  langstiorfii  (Brong.)  Ung. 
Glyptostrobus  europceus  Brong. 
Equisetum  (?)  sp. 

Dicotyledons:  Ficus  (?),  Quercus  (?),  etc. 
Plant  stems. 

Mr.  Knowlton  adds  the  following  remarks  in  the  letter  submitting 
the  results  of  his  examination : 

Neither  of  the  conifers  can  be  relied  upon  to  prove  close  questions  of  age,  for  they 
have  a  considerable  vertical  range,  (ityptotirobus  europfeus  is,  however,  absolutely, 
and  Sequoia  lanijsdorfii  almost  exclusively,  confined  to  the  Tertiary.  Both  species 
have  a  wide  geographical  range,  and  are  very  abundant  forms.  The  first  is  abundant 
in  this  country  in  the  Fort  Union  group,  Upper  Laramie  (Fort  Union)  of  Canada, 
Mackenzie  River,  Alaska,  and  Arctic  Miocene  in  general.  In  Europe  it  is  mainly 
confined  to  the  Eocene  and  Miocene.  Sequoia  laiujsdorjii  has  been  once  reported  from 
the  true  Laramie,  but  I  regard  this  identification  as  extremely  doubtful.  It  is  very 
abundant  in  the  Kort  Union  group,  and  is  also  found  at  the  same  places  as  the  other. 
The  Dicotyledons  are  fragmentary  but  seem  to  belong  to  such  genera  as  Ficus, 
Quercus,  etc.  The  Equisetum  and  vegetable  steins  are  worthless  for  stratigraphic 
purposes. 

The  evidence,  incomplete  as  it  is,  shows  the  Tertiary  age  of  these  beds,  but 
whether  they  are  Eocene  or  Miocene  it  is  impossible  to  say. 

In  the  bluffs  of  Kirtley  Creek,  about  1£  miles  below  the  Lemhi  Placer 
Company's  bar,  is  the  following  exposure: 

Feet. 

At  top  of  bluff,  obscured  beneath  Pleistocene  gravel 2(  > 

Argillaceous  sands 5 

Pure  sands,  concretions  iit  base 10 

Coarse  gravel 10 

Bright-red  clays,  very  pure 5 

Red  clays  with  pebbles  finer  than  in  the  gravel  above 5 

Gravel  probably  still  underlies,  but  it  is  covered. 

The  position  of  the  beds  is  nearly  horizontal,  with  a  possible  slight 
dip  to  west.  Their  relation  to  the  other  beds  in  the  valley  could  not 
at  the  time  be  determined,  but  they  are  quite  likely  younger  than  the 
series  of  green  clays,  at  one  point,  indeed,  apparently  resting  upon 
them  or  a  nearly  allied  stratum.  Moreover,  apparently  the  red  clays 
everywhere  occur  at  a  higher  altitude  than  the  green. 

The  conglomerates  of  this  series  of  Tertiary  beds  outcrop  in  great 
force  about  the  periphery  of  the  Salmon  City  Valley,  especially  at  the 
west  base  of  the  Continental  Divide,  and  are  difficult  of  reference. 
Being  auriferous,  they  are  the  most  important,  from  an  economic  stand- 
point, of  all  the  beds.  They  are  exposed  in  the  cuts  of  the  Lemhi 


F.UIIIIDGK  1 


CENOZOIC    IN    SALMON-LEMHI    VALLEY. 


233 


Placer  Mining  Company  on  Kirtley  Creek,  about  2  miles  from  the  base 
of  the  range.  At  this  point  the  section  given  below  was  obtained. 

The  gravels  of  this  section,  Pleistocene  and  Tertiary,  are  made  up 
of  quart/ites  and  schists  derived  from  the  neighboring  mountains. 
The  two  are  readily  distinguished,  the  Pleistocene  being  loose,  easily 
disintegrated,  and  carrying  in  the  interstices  of  the  bowlders  and  peb- 
bles considerable  earthy  matter;  the  Tertiary  gravels  being  a  compact 
mass  of  large  pebbles,  up  to  1  foot  HI  diameter,  in  a  grit  matrix,  the 
whole  moderately  hard  and  difficult  to  hydraulic. 

The  unconformity  between  beds  c  and  d  is  evidenced  not  only  by  the 
wavy  lines  between  the  two,  as  shown  in  the  sketch,  but  also  by  the  fact 
that  the  gravel  beds  c  rests  successively  upon  several  of  the  underlying 
beds.  Both  series  of  strata,  however,  those  above  and  those  below  the 
line  of  unconformity,  are  alike  flexed  in  the  crumpling  that  has  taken 
place  in  the  region.  The  beds  <i!-/'are  considered  "bed-rock"  by  the 


d.  e_f  g 

FIG.  38  --Section  of  Tertiary  gravels  at  Lemhi  placer  mine. 

a,  Pleistocene  gravel,  overlying  unconformably  the  Tertiary  series,  0-10  feet;  auriferous,  b,  Series 
of  sandstones  with  some  gravelly  layers ;  average  thickness,  30  feet,  c,  Coarse  conglomerate ;  mate- 
rial, quartzite;  20  feet;  unconformable  with  d ;  auriferous,  d.  Yellow  sandstone;  solid  bed,  3 feet; 
leaf-bearing,  e.  White  sandstone;  solid  bed,  5  feet;  leaf-bearing.  /,  Very  white  sandstone;  thin- 
bedded,  flue-grained;  5  feet.  (<t-/,  Bed-rock,  of  miners.)  g.  Series  of  sandstones  and  conglomerates 
in  equal  proportions;  15  feet  show,  but  it  extends  below  the  present  depth  of  the  cut;  auriferous. 

miners,  though  the  series  below  is  said  to  be  auriferous,  even  in  paying 
degree  under  favorable  conditions  of  water  and  work.  The  thickness  of 
the  conglomerate  series  is  unknown ;  it  possibly  reaches  300  or  400  feet. 

The  Pleistocene  cap  to  the  series  is,  in  part,  probably,  wash  direct 
from  the  mountains,  and  in  part,  doubtless,  the  product  of  subaerial 
denudation  of  the  Tertiaries. 

On  the  hills  of  greater  height  on  both  sides  of  Kirtley  Creek,  at  an 
altitude  of  about  200  feet  above  the  stream,  is  a  bed  of  coarse  quartzite- 
con glomerate,  very  hard,  fracturing  across  matrix  and  pebbles  alike. 
Its  relation  to  the  other  beds  was  undetermined. 

It  was  impossible,  within  the  short  time  available,  to  determine  com- 
pletely the  relative  stratigraphic  position  of  the  three  or  four  varie- 
ties of  Tertiary  rocks  described.  The  sandstones  with  their  subordinate 
conglomerate  beds,  which  outcrop  in  the  blufl's  of  the  Salmon  Eiver, 
are  beyond  question  overlain  by  the  green  clays,  and  these  in  turn, 


234  GEOLOGICAL    RECONNAISSANCE    ACROSS    IDAHO. 

perhaps,  by  the  bright-red  clays  and  associated  gravels  outcropping 
in  the  bench  lands  north  of  Kirtley  Creek,  below  the  placer  mines. 
Concerning  the  position  of  the  conglomerate  series  at  the  mines, 
however,  there  is  more  doubt,  for  the  strata  underlying  the  area  to  the 
west  were  covered  for  a  considerable  distance  by  wash.  The  dip  of 
the  conglomerates  at  the  mines  is  15°  to  50°  westward,  and  they  pass 
beneath  the  general  surface  of  the  long,  gentle  valley  slope.  If  they 
are  not  brought  up  by  flexures  within  a  mile  or  two  below  the  Lemhi 
placers,  they  are  succeeded  more  or  less  directly  by  the  green  clays, 
and  would  therefore,  in  this  case,  correspond  to  the  sandstone- 
conglomerate  series  along  the  river  bluffs.  Certain  resemblances 
between  the  two  have  already  been  noticed.  It  is  possible  that  they 
may  wholly  underlie  the  river  outcrops,  the  latter  sandstones  perhaps 
being  concealed  nearer  the  border  of  the  basin  in  the  intervals  of  no 
outcrop. 

The  valley  of  the  Payette  Eiver,  at  Horseshoe  Bend,  also  carries  a 
series  of  intermontane  Tertiary  beds,  the  materials  for  which  were 
doubtless  derived  from  the  inclosing  granite  ranges.  The  series  is  in 
general  a  succession  of  sandstones  with  some  clays  and  conglomerates, 
carrying  plant  remains  which  in  some  instances  have  been  sufficiently 
abundant  to  form  carbonaceous  shales,  in  others  coal  itself.  In  appear- 
ance the  Payette  beds  resenible  those  at  Salmon  City;  in  composition 
and  their  place  of  derivation  they  may  be  more  closely  affiliated  with 
the  Snake  Eiver  series.  Of  their  age,  however,  except  that  they  are 
Tertiary,  no  evidence  was  obtained. 

PLEISTOCENE. 

This  consists  of  gravels  occurring  along  stream  bottoms,  of  the  prod- 
acts  of  subaerial  disintegration,  or  of  morainal  de"bris.  These  deposits 
will  not  be  discussed  beyond  mention  in  connection  with  other  points 
in  the  report. 

ERUPTIVE  ROCKS. 

The  eruptive  rocks  of  southern  Idaho  present  a  great  variety  of 
types,  and  their  manner  of  occurrence  ranges  from  a  simple  dike  to  a 
series  of  sheets  surrounding  a  probable  volcanic  center  or  neck.  The 
flows  have  probably  taken  place  at  many  stages  in  the  development  of 
the  region,  from  Archean,  possibly,  to  late  Tertiary,  and  even  at  the 
present  day  traces  of  volcanic  activity  remain  in  innumerable  hot 
springs  throughout  the  State. 

Along  the  front  of  the  Boise  Eange  are  eruptives  of  a  number  of 
types  in  dikes,  irregular  masses,  and  sheets. 

Among  the  dikes  and  irregular  masses  is  a  boss  of  andesite  in  the 
bluffs  on  the  north  side  of  the  Boise  Valley,  a  mile  above  Boise.  Prior 
to  the  erosion  of  the  bluffs  this  andesite  was  overlain  by  the  Tertiary 
sediments,  in  which  there  was  a  slight  upward  bowing.  A  portion  of 
the  andesite  has  been  identified  as  of  the  augitic  variety;  it  is  dark- 


ELURiuoK.]  ERUPTIVES   OF    BOISE    KANGE.  235 

gray  and  banded.  The  associated  variety  is  light  pink-gray  and  vesic- 
ular. The  groundmass  is,  possibly,  a  devitrifled  glass,  throughout 
which  are  small  crystals  of  decomposing  triclinic  feldspar,  and  some 
magnetite.  The  interrelation  of  the  two  andesites  was  not  determined. 

About  5  miles  east  of  Boise,  in  the  granite  of  the  Boise  Range,  near 
the  edge  of  a  prominent  basalt  flow,  in  the  Flannigan  prospect  tunnels, 
are  several  narrow  dikes  of  aphanitic  syenite  (Itunprophyre),  having  a 
general  trend  of  N.  50°  to  70°  W.,  with  a  dip  of  45°  to  80°  SW.,  appar- 
ently coincident  with  the  foliation  of  the  granite.  A  vein  of  quartz 
with  a  slight  admixture  of  granitic  material  lies  between  two  of  the 
dikes,  at  a  little  distance  from  each,  and  is  said  to  be  gold  bearing. 

On  the  Boise- Idaho  City  stage  road,  from  2  to  3  miles  above  Boise, 
and  just  at  the  edge  of  the  granite  mountain  slope,  is  an  outcrop  of 
rhyolites,  of  an  area  somewhat  less  than  a  square  mile.  The  variety 
of  rock  most  typical  of  the  locality  is,  perhaps,  a  microspherulitic  rhyo- 
lite,  light-gray  to  faint  pink,  with  small  porphyritic  crystals  of  sanidine 
and  some  quartz.  This  alternates  with  a  mottled  variety,  pink  and 
dark  gray,  with  the  same  porphyritic  development  of  crystals.  Occa- 
sionally the  groundmass  is  microcrystalline  or  cryptocrystalline,  but 
it  is  more  frequently  glassy.  Perlitic,  spherulitic,  and  vesicular  vari- 
eties also  occur.  Fluidal  structure  is  common  throughout  the  entire 
mass.  The  rhyolite  cuts  the  gray  granite  of  the  Boise  Range,  form- 
ing an  apparently  irregular  body,  the  maximum  dimension  having  a 
northwest-southeast  direction. 

About  0  miles  northeast  of  Boise,  and  north  of  the  Boise-Idalio  City 
road,  in  the  slopes  of  the  range,  is  a  dike  of  quartz-porphyry  running 
through  the  granite  in  a  general  northwest-southeast  direction.  The 
dike  is  not  over  3  or  4  feet  wide,  and  with  the  granite  on  either  side 
has  been  prospected  for  gold. 

A  hundred  yards  west  of  the  above  outcrop,  and  also  at  two  or  three 
other  points  on  the  lower  slopes  of  the  Boise  Range,  notably  one  a  little 
south  of  the  Boise-Idaho  City  road,  about  2  miles  from  Boise,  are  deposits 
of  a  brown,  porous,  stratified,  rhyolitic  tuff,  carrying  a  large  amount  of 
angular  and  subangular  granitic  debris,  in  fragments  up  to  1  or  2  feet 
in  diameter.  The  deposit  occurs  at  various  altitudes,  but  its  relations 
to  the  Tertiary  beds  of  the  region  are  not  clearly  shown. 

One  of  the  most  widely  distributed  of  the  eruptives  is  basalt.  This 
forms  a  large  part  of  the  floor  of  the  Snake  Plains,  and  occurs  in 
isolated  sheets,  bosses,  and  dikes  at  many  points  in  the  mountains. 
Prominent  among  these  are  two  or  three  independent  sheets  on  the 
western  slopes  of  the  Boise  Range;  an  enormous  flow  along  the  South 
Fork  of  the  Boise  River  within  the  mountains;  others,  very  heavy, 
above  the  three  forks  of  Carnas  Creek,  a  tributary  of  the  Middle  Fork 
of  the  Salmon;  and  several  along  the  Salmon  River  between  Salmon 
City  and  Challis.  Of  those  on  the  western  slopes  of  the  Boise  Range 
a  typical  occurrence  is  a  remnant,  a  half  mile  in  area,  about  5  miles 


236  GEOLOGICAL    RECONNAISSANCE    ACROSS    IDAHO. 

east  of  Boise,  1,000  or  1,500  feet  above  the  valley.  The  flow  filled  the 
early  gulches  and  capped  the  intervening  ridges,  its  surface  acquiring 
a  position  of  rest  but  slightly  at  variance  with  the  general  slope  of  the 
range.  In  later  times  the  gulches  have  been  recut  to  the  granite 
beneath.  Beyond  the  present  limits  of  the  flow  no  trace  of  connecting 
bed  of  basalt  could  be  found;  it  was  probably,  therefore,  a  small 
isolated  body,  welling  up  through  a  local  dike. 

Between  Slaters  Creek  and  the  Boise  the  basalt  constitutes  the  floor 
of  a  high  mesa  which  extends  for  10  to  15  miles  along  the  base  of  the 
mountains,  with  the  appearance  of  having  at  oue  time  been  continuous 
with  the  lava  plains  of  the  Snake  Valley.  The  mesa  has  a  gentle  slope 
of  2°  or  3°  to  the  southwest,  the  basalt  resting  directly  upon  granite. 
In  the  gorge  of  the  Boise  the  basalt  forms,  for  4  or  5  miles  from  the 
mouiitains,  sharp  vertical  walls,  the  columnar  structure  being  well 
developed  on  both  upper  and  under  surfaces  of  the  flow.  The  thick- 
ness of  the  sheet  varies  from  point  to  point,  averaging  perhaps  75  or 
100  feet. 

Within  the  mountains  the  basalt  follows  the  main  Boise  to  the  South 
Fork,  passing  up  this  nearly  to  Pine  Grove,  a  few  miles  northeast  of 
Little  Carnas  Prairie.  It  is  also  reported  on  Moores  Creek  nearly  to 
Idaho  City.  None  is  said  to  occur  on  the  main  stream  above  the  mouth 
of  South  Fork.  There  appear  to  have  been  at  least  two  distinct  periods 
of  flow,  the  remnants  of  which  are  attached  to  the  canyon  walls,  and 
lie,  one,  500  to  1,000  feet  above  the  present  stream  level,  the  other  but 
50  to  200  feet  above.  Which  is  the  older  was  not  determined.  Whether 
it  would  be  possible,  by  descending  the  canyon  from  Pine  Grove,  to 
trace  the  respective  flows  continuously,  or  at  least  to  trace  a  particular 
flow  entirely  through,  can  not  be  stated,  but  it  is  a  peculiar  feature  of 
their  occurrence  that  the  different  benches  of  basalt  are  everywhere 
found  practically  at  about  the  same  height  above  the  present  water 
level ;  they  fall  with  the  stream,  impelling  the  belief  that  the  lava  once 
flowed  through  the  canyon  as  does  the  stream  of  to  day. 

Smith  and  Little  Camas  prairies  are  two  broad  intermontane  flats 
on  the  course  of  the  South  Fork,  the  sites  of  ancient  valleys  refilled  to 
their  present  levels  with  basalt  of  the  same  flows  as  in  the  canyon. 
The  canyon  is  recut  to  a  depth  of  1,000  to  1,500  feet,  presenting  cross- 
sections  of  the  valleys,  which,  as  regards  the  basalt,  are  the  same  as 
those  of  the  canyon  in  general.  The  precise  nature  of  the  two  great 
lava  lakes  is  unknown.  They  are  evidently  near  the  source  of  the 
basalt,  and  the  valleys  filled  by  them  seem  to  have  been  the  receiving 
reservoirs,  in  part  for  a  stream  from  a  source  farther  up  the  Boise,  in 
part  for  flows  from  fissures  in  some  of  the  side  valleys  tributary  to  the 
basins,  and,  perhaps,  at  the  outset,  from  fissures  in  the  bottoms  of  the 
valleys  themselves.  From  the  lakes  the  flows  continued  down  the 
early  Boise  Canyon  to  the  open  valley  of  the  Snake,  receiving  from 
the  tributaries  on  either  side  greater  or  less  accessions.  This  was 


BLUBIDOE.)  ERUPTIVES    FROM   BOISE    TO    ROCKY    BAR.  237 

eminently  the  case  at  Moores  Creek,  where  the  flows  are  very  heavy. 
Some  of  the  smaller  access  ous  were  doubtless  from  the  valley  of  Rat- 
tlesnake Creek,  two  irregular  and  somewhat  extensive  basalt  dikes 
occurring  about  the  forks  2  miles  above  the  mouth.  Just  below  the 
forks  of  Smith  Creek,  also,  is  another  dike  of  the  same  composition  as 
the  foregoing,  but  with  diabasic  structure.  Indeed,  in  the  region 
crossed  by  trail  from  Boise  to  the  Trinity  Lakes  and  Eocky  Bar,  basalt 
and  diabase  dikes  are  numerous.  The  most  isolated  occurrence  of 
diabase  is  as  a  complex  near  the  head  of  Falls  Creek,  outcropping  on 
the  lower  slopes  of  the  Trinity  Range.  The  outcrop  is  several  hundred 
feet  across,  of  irregular  outline,  and  forms  a  conspicuous  red  and  blue 
patch  in  the  gray  of  the  mountain  side.  The  dikes,  both  of  basalt  and 
diabase,  all  cut  gray  granite. 

A  gray  quartz-porphyry  showing  a  groundmass  of  micropegmatite 
with  porphyritic  crystals  of  feldspar  was  observed  on  the  Boise  from  a 
point  2  or  3  miles  below  the  entrance  of  the  South  Fork,  to  Trail  Creek. 
Its  extent  beyond  this  is  undetermined. 

On  the  ridge  north  of  the  stream  flowing  from  the  Trinity  Lakes  the 
granite  is  cut  by  a  body  of  quartz-diorite  porphyrite,  of  a  finely  crystal- 
line matrix  and  porphyritic  feldspar.  Between  here  and  Rocky  Bar,  a 
distance  of  8  or  9  miles,  there  are  several  occurrences  of  this  rock  or 
the  quartzless  variety,  diorite-porphyrite.  Within  the  same  region, 
also,  are  occasional  dikes  of  basalt  or  augite-andesite,  which,  was  not 
determined.  In  the  Red  Warrior  camp,  2  miles  southwest  of  Rocky 
Bar,  is  also  a  light-gray  to  white,  finely  crystalline  or  microcrystalline 
quartz-porphyry,  similar  to  the  Leadville  porphyry.  Grains  of  pyrite 
occur  disseminated  through  it.  The  specimen  examined  was  from  the 
dump  of  an  inaccessible  tunnel  by  the  roadside,  a  mile  or  so  northwest 
of  the  center  of  the  camp.  From  the  quantity,  one  may  infer  it  to  be 
a  rock  of  importance,  at  least  for  this  mine. 

In  the  Jim  Elaine  mine,  a  mile  and  a  half  east  of  Rocky  Bar,  there 
occurs  a  dike  of  basalt  carrying  a  few  rounded  quartz  grains.  Its  rela- 
tion to  the  ore  body  was  not  altogether  clear. 

On  the  eastern  side  of  the  high  divide  between  Rocky  Bar  and  Atlanta, 
on  the  lower  half  of  the  slope,  there  occurs  in  the  granite  a  succession 
of  enormous  dikes,  100  to  200  feet  wide,  of  quartz-porphyry  and  a  quartz- 
bearing  syenite-porphyry,  which  extends  northwest  several  miles  across 
country.  What  were  probably  similar  occurrences  were  seen  at  a  dis- 
tance in  several  of  the  ranges  about  Atlanta,  but  the  localities  were  not 
visited.  The  prevailing  trend  of  the  ledges  is  east  and  west,  a  direc- 
tion often  observed  also  in  the  granite  and  ore  bodies  of  the  Rocky 
Bar  and  Atlanta  districts.  The  dikes  dip  45°  or  50°  N.  to  vertical. 
Between  them  is  the  ordinary  granite  of  the  country,  often  appearing 
like  included  masses.  The  quartz- porphyry  is  typical,  a  felsitic  ground- 
mass,  with  small  irregular  bodies  of  quartz  and  occasional  small  crystals 
of  feldspar,  the  general  tint  of  the  rock  being  a  decided  pink.  In  the 


238  GEOLOGICAL   RECONNAISSANCE   ACROSS    IDAHO. 

quartz-syenite-porphyry,  also  pink  in  effect,  the  quartz  is  distributed 
sparingly  through  a  finely  crystalline  groundmass,  while  the  orthoclase 
is  developed  in  prominent  porphyritic  crystals.  The  basic  mineral  is 
usually  finely  crystalline,  and  in  aggregates  mottles  the  rock  a  dark- 
green.  The  relation  between  the  quartz-porphyry  and  the  quartz- 
bearing  syenite-porphyry  was  not  determined,  although  they  evidently 
occur  in  close  association  with  each  other. 

In  Montezuma  Gulch,  east  of  Atlanta,  between  a  third  and  half  the 
distance  to  the  head,  is  a  narrow  dike  of  aphanitic  syenite  (lampro- 
phyre)  similar  to  that  occurring  in  the  Flannigan  mines  on  the  west 
slope  of  the  Boise  Range.  Its  lineal  extent  is  unknown. 

In  the  Monarch  mine,  on  Atlanta  Hill,  is  a  narrow  dike  of  probable 
decomposed  syenite  (lamprophyre),  but  the  specimen  collected  was  too 
much  altered  to  permit  satisfactory  determination. 

In  the  Tahoma  mine  is  a  dike  of  white,  decomposed  porphyry,  also 
altered  beyond  identification.  Its  thickness  varies  from  25  to  50  feet 
on  the  several  levels.  In  one  or  two  places  the  vein  has  been  slightly 
thrown  by  it.  The  general  trend  of  the  dike  is  N.  26°  W.;  the  dip, 
southwest.  All  the  eruptives  of  the  Atlanta  district  not  infrequently 
contain  pyrite  in  minute  crystals;  the  influence  of  the  dikes  upon  the 
mineralization  of  the  veins,  however,  is  undetermined. 

In  the  granites  of  the  Sawtooth  Range,  on  both  the  east  and  west 
sides,  are  frequent,  narrow,  fine  grained  dikes  of  aphauitic  syenite 
(lamprophyre),  and  what  is  likely  a  nearly  related  rock  with  porphy- 
ritic crystals  of  triclinic  feldspar.  Quartz-porphyry  was  also  observed 
on  the  east  side  of  the  range  in  the  slopes  west  of  Upper  Redfish  Creek. 
A  large  body  of  this  rock  appears  in  a  side  gulch  entering  the  main 
valley  about  a  mile  above  the  lake,  and,  from  their  appearance  at  a 
distance,  similar  bodies  are  believed  to  exist  at  various  points  along 
the  range  front. 

Rhyolite  (aporhyolite)  occurs  as  a  large  irregularly  shaped  dike  in  a 
hill  on  the  east  side  of  the  Salmon  Valley,  10  to  12  miles  above  the 
sharp  bend  of  the  river  to  the  east  and  4  to  5  miles  above  the  Lower 
Redfish  Lake.  The  rock  is  banded  gray  and  brown,  the  brown  often 
having  a  glassy  or  resinous  appearance,  the  gray  being  very  close 
textured,  and  marked  with  fine  striations,  as  though  of  flow  structure. 
The  prevailing  rock  of  the  country  is  the  usual  gray  granite. 

In  the  morainal  debris  about  the  Lower  Redflsh  Lake,  derived  from 
the  Sawtooth  Range,  besides  the  more  common  pinkish  granite,  there 
are  numerous  bowlders  of  quartz-diorite-porphyrite — a  dark-gray  rock 
with  porphyritic  plagioclase  and  much  sanidine. 

A  large  body  of  quartz-porphyry  of  greenish-gray  grouudmass  with 
feldspar  in  prominent  crystals  forms  a  high  hill  in  the  forks  of  Valley 
and  Stanley  creeks.  The  general  trend  of  the  mass  is  north  and  south. 
On  the  east  side  of  Valley  Creek,  about  4  miles  above  the  mouth  of 
Stanley,  is  a  prominent  body  of  quartz  bearing  syenite- porphyry,  the 


EI.DKIUUE.J  ERUPTIVES    OF    THE    SAWTOOTH    RANGE.  239 

groundmass  of  which  is  micropegmatite  with  a  very  small  amount  of 
quartz.  The  rock  is  greenish-yellow.  The  porphyritic  constituent  is 
orthoclase.  Near  the  head  of  Valley  Creek,  by  the  roadside,  are 
numerous  small  dikes  of  dark-gray,  altered  diabase,  cutting  granite. 

The  main  Sawtooth  Range  was  crossed  only  at  one  point,  at  the  head 
of  Upper  Redflsh  Creek;  but  in  the  occurrence  of  eruptives  in  the 
debris  from  the  portion  north  of  this,  and  in  the  veinings  that  may  be 
observed  from  a  distance,  there  are  evidences  of  a  number  of  varieties, 
among  which  the  quartz-diorite-porphyrite  above  described  is  an 
important  type.  The  veinings  mentioned  may  be  seen  in  all  parts  of 
the  main  range,  no  was  the  chief  mass  of  one  of  the  rugged  peaks,  now 
in  a  broad  band  of  color,  running  along  the  precipitous  sides  in  contact 
with  a  pink-colored  granite  which  here  seems  to  constitute  the  mass  of 
the  range.  North  of  Cape  Horn  the  range  has  the  appearance  from  a 
distance  of  returning  to  the  gray  granite  as  its  chief  constituent — the 
granite  which  occurred  at  the  southern  end  where  crossed  and  which  is 
the  prevailing  rock  of  the  great  mass  of  mountains  to  the  west.  East 
of  the  Sawtooth  the  gray  granite  passes  into  the  high  range  forming 
the  divide  between  the  main  Salmon  and  its  Middle  Pork,  and  into  the 
mountains  east  of  the  Salmon  River  and  Valley  Creek.  In  nearly 
every  portion  of  its  area,  however,  it  is  frequently  cut  by  eruptives  of 
one  kind  or  another. 

East  of  the  trails  to  Sheep  Mountain  and  Loon  Creek,  in  the  direc- 
tion of  Bonanza,  great  bodies  of  eruptives  appear  in  the  lofty  peaks 
and  intricate  ranges  there  present.  These  were  all  beyond  the  route 
of  reconnaissance.  On  the  line  of  exploration,  however,  on  the  several 
heads  of  Beaver  Creek,  and  in  the  gulch  leading  down  to  the  mining 
camp  of  Sea  Foam,  are  numerous  dikes  of  diorite-porphyrite,  crystal- 
line-granular throughout,  with  porphyritic  development  of  the  feld- 
spar. On  Beaver  Creek,  also,  is  a  white  or  light-gray,  fine-grained 
variety  of  quartz-porphyry  of  frequent  recurrence;  magnesian  mica  is 
quite  prominent  throughout  this  rock.  Other  quartz-porphyries  differ- 
ing somewhat  from  the  foregoing  in  the  amount  of  quartz  or  the  por- 
phyritic development  of  their  feldspar  appear  in  dikes  of  greater  or 
less  extent  in  the  mountains  between  Beaver,  Bernard,  and  Loon 
creeks.  On  the  latter  stream  they  are  particularly  abundant.  They 
all  cut  the  granite  of  the  region. 

In  the  canyon  of  LOOD  Creek,  3  miles  below  Oro  Grande,  as  dikes  in 
a  coarse,  pink  granite,  occurs  a  finely  crystalline  quartz-porphyry  of  a 
color  closely  resembling  that  of  the  granite.  The  orthoclase  of  the 
eruptive  may  be  porphyritic,  while  the  quartz  usually  occurs  in  minute 
grains  in  the  groundmass.  Both  granite  and  eruptive  are  conspicuous 
features  of  the' region's  geology. 

In  the  same  vicinity  occurs  also  a  dark-gray  quartz-diorite-porphyrite 
of  a  totally  different  facies  from  the  pink.  The  crystallization  of  this  is 
in  two  or  three  degrees  of  coarseness,  white  feldspar  crystals  being 


240  GEOLOGICAL    RECONNAISSANCE    ACROSS    IDAHO. 

prominent,  though  not  large.  The  rock  is  of  extensive  outcrop,  and 
occurs  in  heavy  masses,  in  places  apparently  resting  upon  the  granite 
of  the  region ;  but  such  an  appearance  may  be  due  to  an  inclined  posi- 
tion of  the  dikes  in  the  granite  and  to  the  relative  position  of  their  out- 
crops in  the  precipitous  walls  of  the  canyon.  The  rock  again  appears 
about  3  miles  farther  down  the  canyon,  from  which  point  it  may  be.  fol- 
lowed in  continuous  outcrop  for  a  distance  of  8  miles  or  more  down- 
stream— beyond  Beaver  Ranch — and  to  the  east  far  toward  the  summit 
of  the  high  divide  between  the  waters  of  Loon  and  Cainas  creeks. 

On  the  east  side  of  this  divide,  along  the  West  Fork  of  Gamas  Creek 
and  on  the  western  branches  of  the  Middle  Fork,  is  another  dark  gray 
porphyrite  with  many  feldspar  and  quartz  crystals  and  containing 
numerous  small  fragments  of  dark-green  rocks.  A  Huidal  structure  is 
clearly  shown  under  the  microscope,  though  in  the  baud  specimen  it 
would  hardly  be  suspected.  The  rock  forms  the  chief  outcrop  in  the 
region  mentioned,  but  a  gray  rhyolitic  tuft'  is  also  present  at  intervals. 
Well  down  on  the  West  Fork  of  Camas  Creek  is  also  a  yellow  to  yel- 
lowish-red quartz-porphyry,  outcropping  in  three  or  four  heavy  ledges 
50  to  100  feet  wide.  This  porphyry  is  in  part  solid,  in  part  somewhat 
vesicular.  Apparently  it  cuts  the  gray  rhyolite,  but  their  relations 
were  indistinct. 

On  passing  from  the  area  of  gray  rhyolite  and  associated  quartz- 
porphyry  near  its  eastern  edge,  there  appears  at  a  point  a  mile  and  a 
half  above  the  mouth  of  the  West  Fork  of  Camas  Creek  a  second 
prominent  series  of  lighter-colored  rhyolites  and  rhyolitic  tuffs  in 
heavy  beds  striking  about  N.  15°  E.  and  dipping  30°  E.  These  beds 
are  continued  far  into  the  mountains  both  north  and  south,  and  in  a 
northern  direction  there  appears  at  a  distance  a  tendency  in  the  strike 
to  bend  to  the  northwest,  as  though  the  series  formed  the  eastern  edge 
of  a  dome-shaped  mass  of  eruptives.  This,  however,  is  indefinite.  The 
relation  of  this  second  series  to  the  dark-gray  rhyolites  and  quartz- 
porphyries  is  unknown,  their  contact  being  covered  at  the  point  trav- 
ersed, but  it  is  quite  possible  that  the  tuff  rests  upon  the  gray  rhyolites 
and  is  in  turn  overlain  by  the  lighter  rhyolites  with  which  it  is  asso- 
ciated. The  tuff  is  bedded,  and  consists  of  a  soft,  white,  green,  or 
purple  matrix,  with  included  angular  fragments  of  rhyolite  from  a  half 
inch  to  2  feet  in  diameter.  It  is,  at  a  rough  estimate,  200  to  300  feet 
thick,  and  extends  along  the  creek  a  distance  of  one-quarter  mile. 

Banded  rhyolite  of  somewhat  varying  texture  and  solidity  apparently 
overlies  the  tuff,  occurring  in  nearly  continuous  outcrop  for  a  half  mile 
farther  downstream.  Other  tuffs  succeed,  with  their  included  rhyolite 
fragments,  and  a  matrix  rather  less  friable  than  that  of  the  lower 
beds,  forming  the  last  outcrops  along  the  West  Fork.  Across  main 
Camas  Creek  from  the  foregoing  series,  however,  and  just  below  the 
Three  Forks,  is  a  very  similar  succession  of  outcrops,  rhyolites  and 
rhyolitic  tuffs  in  alternating  bands  500  to  600  feet  thick,  often  repeated, 


KLURIDOE.]  EEUPTIVES   ALONG    CAMAS    CREEK.  241 

striking  about  N.  05°  E.,  with  a  dip  of  10°  to  20°  SE.  Although  this 
series  is  similar  to  that  ou  the  West  Fork  and  may  be  the  same,  direct 
continuity  was  not  shown.  This  series  is  continued  with  variation 
to  the  head  of  the  East  Fork  of  Camas  Creek  (Silver  Creek),  and  be- 
yond into  the  ranges  bordering  Prairie  Basin  on  the  west.  Ou  Silver 
Creek  several  varieties  of  the  rhyolite  appear,  together  with  other  rocks 
more  or  less  nearly  related.  Most  of  them  occur  in  heavy  ledges,  walling 
in  the  canyon  with  ragged,  precipitous  sides.  On  the  north  side  of  Sil- 
ver Creek,  a  N.  60°  E.  strike  and  northwest  dip  are  maintained  by  the 
eruptives  for  a  distance  of  7  or  8  miles  up  the  creek.  On  the  south 
side,  in  the  high  ridge  southeast  of  the  Three  Forks,  the  general  dip  is 
southward,  with  more  or  less  gentle  flexing.  On  the  upper  portion  of 
the  creek  strike  and  dip  vary  somewhat. 

Of  the  foregoing  rocks,  the  tuff  is  of  widest  occurrence,  and  on  both 
sides  of  the  divide  between  Silver  and  Panther  creeks  it  occurs  in 
heavy  beds,  underlying  the  summit  itself. 

The  Middle  Fork  of  Camas  Creek  was  not  examined.  From  the 
Three  Forks,  however,  there  could  be  seen  a  few  miles  up,  between  the 
two  chief  branches,  an  enormous  butte  of  eruptive  rock,  appearing  like 
the  remnant  of  a  great  lava-flow.  It  is  of  a  very  dark-brown  color,  and 
at  a  distance  resembles  basalt  At  other  points  in  this  canyon  the 
eruptives  of  the  East  and  West  forks  seem  to  prevail. 

In  the  gravel  wash  in  the  vicinity  of  the  Three  Forks  were  two  varie- 
ties of  rhyolite  (aporhyolite)  resembling  that  found  in  the  upper  valley 
of  the  Salmon  River.  These  may  have  been  brought  down  any  of  the 
streams. 

From  the  foregoing  it  is  evident  that  the  region  about  the  Three  Forks 
of  Camas  Creek  for  a  radius  of  15  to  25  miles  has  at  various  intervals 
in  the  past  been  one  of  intense  eruptive  activity.  The  series  of  rhyo- 
lites  and  tuft's  extends  entirely  across  the  drainage  system,  and  from 
the  high  divide  on  its  south  to  fully  10  miles  below  the  junction  of  the 
three  main  creeks.  Beyond  this  area,  particularly  to  the  west,  rhyolites 
and  other  eruptives  still  occur  over  vast  areas ;  to  the  east,  while  pres- 
ent, it  is  not  to  such  complete  exclusion  of  the  primary  rocks  of  the 
country. 

The  high  mountains  forming  the  divide  between  Prairie  Basin  and 
Yellow  Jacket  Creek,  a  tributary  entering  Camas  Creek  several  miles 
north  of  the  Three  Forks,  are  largely  composed  of  granite  and  of 
quartzitic  and  micaceous  schists  having  a  prevailing  strike  between 
N.  30°  E.  and  N.  20°  W.,  and  a  dip  generally  away  from  the  center  of 
the  range,  though  often  very  steep  or  even  vertical.  The  slates,  which 
alone  were  crossed  on  the  route  traveled,  are  frequently  cut  by  erup- 
tives, in  some  instances  at  angles  with  the  strike  and  dip,  in  others 
coincident  with  them. 

Among  the  eruptives  are  the  following :  At  the  entrance  to  Fourth 
of  July  Canyon,  a  massive  body  of  pink,  banded  rhyolite,  in  appearance 
16  GEOL,  PT  2 16 


242      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

a  sheet  50  to  60  feet  thick,  with  a  strike  north  and  south  and  dip  E.  20°, 
about  that  of  the  schists  of  the  locality.  The  sheet  extends  for  several 
miles  along  the  eastern  front  of  the  range,  with  little  apparent  interrup- 
tion. About  2  miles  above  the  mouth  of  the  canyon  is  a  second  dike 
of  rhyolite,  20  feet  wide,  with  finely  developed  dull-yellow  feldspar 
crystals,  one-half  to  1  inch  long.  This  is  confined  between  slates,  with 
about  the  same  strike  as  they,  but  whether  with  coincident  dip  is  uncer- 
tain. One  mile  above  this  is  another  eruptive  of  similar  nature,  strik- 
ing N.  25°  W.,  cutting  through  the  slates.  The  summit  of  the  divide 
carries  a  prominent  outcrop  of  andesite,  containing  hornblende,  mica, 
and  augite.  This  rock  is  pink  and  coarse  grained,  with  a  white  feld- 
spar particularly  conspicuous,  abundant,  and  evenly  distributed.  The 
outcrop  extends  for  a  considerable  distance  along  the  range,  and  has 
somewhat  the  appearance  of  an  irregularly  shaped  dike,  or,  in  places, 
of  a  sheet  lying  upon  the  summit  of  the  range.  About  a  mile  and  a 
half  down  the  western  slope  of  the  divide,  on  the  trail,  is  a  narrow 
dike  of  coarse,  yellowish-gray  quartz-porphyry  with  much  orthoclase 
in  crystals  averaging  an  inch  in  length,  many  of  them  twinned  on 
the  Carlsbad  type.  A  trachyte,  finely  crystalline  and  yellowish-gray, 
occurs  just  beside  the  porphyry  and  to  the  west  of  it,  of  apparently 
about  the  same  width  as  the  latter.  A  second  dike  of  trachyte,  similar 
to  the  first,  about  15  feet  wide,  occurs  2  or  2£  miles  lower  down  the 
valley,  striking  N.  15°  W.  and  dipping  W.  70°.  The  inclosing  quartz- 
ites  here  dip  70°  E.,  but  it  is  not  certain  that  the  dike  cuts  them  at  all 
points.  Still  another  dike,  of  dark-gray  quartz-porphyry  carrying 
hornblende  and  biotite  with  microspherulitic  groundmass,  occurs  just 
above  the  confluence  of  the  two  forks  of  Yellow  Jacket  Creek.  The 
foregoing  eruptives  all  cut  quartzite  and  mica-slates  of  the  same  kind 
as  those  on  the  east  side  of  the  range.  The  strike  of  the  schists  on  the 
west  side  of  the  range  varies  from  N.  15°  to  75°  W.,  with  westerly  or 
southerly  dip.  The  eruptives  seem  generally  to  follow  them  in  strike, 
but  in  dip  they  sometimes  have  the  appearance  of  cutting  across 
the  stratification.  On  the  main  Yellow  Jacket  Creek,  between  three- 
quarters  of  a  mile  and  2  miles  above  Yellow  Jacket  camp,  are  numerous 
dikes  of  rhyolite  similar  to  that  at  the  entrance  to  Fourth  of  July  Can- 
yon. The  dikes  form  enormous  outcrops  on  the  mountain  sides,  and 
seem  to  have  a  general  trend  N.  15°  E.  with  a  westerly  dip  of  50°  or 
more.  The  dikes  cut  the  quartzites  of  the  country,  and  are  locally  so 
numerous  that  the  slates  appear  as  included  bodies. 

The  eruptive  rocks  of  the  immediate  Yellow  Jacket  mining  district 
embrace  the  following:  A  quarter  mile  above  the  camp,  syenite,  a  fine- 
grained, yellowish-gray  rock,  occurring  apparently  as  a  thin  band  (dike) 
between  strata  of  the  schists,  which  constitute  the  country  rock,  and 
with  them  striking  N.  15°  E.  and  dipping  W.  45°.  Dikes  of  quartz- 
porphyry  occur  between  several  of  the  veins  in  the  Columbia  ground. 
They  are  usually  narrow,  but  of  undetermined  lineal  extent.  This  rock 


ELDRIDOE.]  ERUPTIVES    OF   YELLOW   JACKET    DISTRICT.  243 

is  locally  termed  "porphyry"  by  the  miners.  A  third  rock,  which  occurs 
in  numerous  narrow  dikes  throughout  the  region,  is  a  dark-gray  mica- 
diorite.  A  fourth  and  oft-recurring  eruptive  is  a  syenitic  variety  rich  in 
mica,  known  as  minette;  and  a  fifth,  more  hornblendic,  is  almost  identi- 
cal with  the  rocks  mentioned  above  as  aphanitic  syenite.  These  rocks 
are  typical  of  the  complex  group  called  lamprophyres  by  some  petrog- 
raphers.  Their  occurrence  here  is  in  various  dikes,  and  they  are 
regarded  by  the  miners,  whether  correctly  or  not  the  writer  can  not  say 
on  personal  observation,  as  indicative  of  an  ore  body  near  at  hand.  In 
local  parlance  they  are  termed  "  syenite."  Usually,  so  far  as  opened, 
when  occurring  next  to  the  veins  they  form  their  foot  walls,  while  the 
quartzite  of  the  country  or  one  of  the  other  eruptives  usually  forms 
the  hanging  walls.  A  sixth  eruptive,  capping  the  high  ridge  northwest 
of  the  Columbia  mines,  is  a  very  coarse  quartz-porphyry,  of  the  same 
general  character  as  that  a  mile  and  a  half  west  of  the  Panther- Yellow 
Jacket  divide.  A  seventh  rock  is  an  altered  diabase,  observed  as  a 
dike  in  the  Yellow  Jacket  mine.  The  general  trend  of  the  dikes  is 
with  the  strike  of  the  slates  and  main  veins,  N.  60°  E.,  with  perhaps 
occasional  local  variations. 

On  Panther  Creek,  about  a  mile  and  a  half  below  Fourth  of  July 
Canyon,  is  a  small  deposit  of  a  soft,  white  tuft'  containing  angular  frag- 
ments of  rhyolite  and  possibly  other  eruptives.  Its  derivation  is 
unknown.  Another  and  more  extended  area  of  the  same  nature  occurs 
on  Napias  Creek  a  little  below  California  Bar,  where  it  forms  the  hills 
both  north  and  south  of  the  stream  for  a  mile  or  more  back.  On  Napias 
Creek,  about  2  miles  above  its  confluence  with  Panther* or  Big  Creek, 
are  coarse-grained  syenites  of  two  somewhat  different  types.  They 
occur  in  apparently  heavy  bodies  of  undetermined  outline  in  the  bird's- 
eye  granite  of  the  region. 

A  half  mile  above  California  Bar,  on  the  northern  side  of  the  creek, 
is  a  small  dike  of  either  pyroxene-andesite  or  basalt,  green  and  pink  in 
color,  the  latter  probably  representing  a  stage  of  decomposition.  Other 
eruptives  occur  in  heavy  masses  in  the  vicinity  of  California  Bar,  but 
time  did  not  permit  examination. 

From  the  Salmon  City  Valley  to  the  Pahsimeri,  the  canyon  of  the 
Salmon  is  for  long  intervals  cut  through  eruptives  closely  resembling 
in  composition  and  occurrence  the  series  of  rhyolites  in  the  Three 
Forks  region.  At  the  entrance  to  the  canyon,  about  6  miles  above 
Salmon  City,  are  beds  of  fine  rhyolitic  tuff,  yellow,  green,  and  pink, 
dipping  10°  or  so  north,  downstream.  Underlying,  and  forming  the 
actual  entrance  to  the  canyon,  are  500  to  1,000  feet  of  pink  and  gray, 
banded  rhyolite,  somewhat  spherulitic,  and  of  varying  hardness.  The 
dip  is  15°  to  20°,  its  direction  varying  between  northeast  on  the  east 
side  of  the  river  and  northwest  on  the  west  side.  In  fact,  from  their 
position,  the  rocks  of  this  series  appear  to  belong  to  a  volcanic 
dome  the  center  of  which  is  at  some  point  considerably  to  the  south. 


244  GEOLOGICAL   RECONNAISSANCE   ACROSS   IDAHO. 

Beneath  the  rhyolite,  with  same  dip,  is  an  equally  thick,  very  dense, 
dark  gray,  bauded  rock,  probably  augite-andesite.  Its  exterior  is 
usually  green  or  pink,  from  the  weathering  of  some  contained  mineral, 
possibly  of  chloritic  nature.  About  3  miles  farther  up  the  canyon  the 
andesite  is  followed  by  a  second  rhyolite,  which  appears  from  beneath 
the  former  in  a  great  half  dome,  dipping  north.  The  rock  is  light  gray, 
with  a  vitreous  groundmass,  and,  like  the  other  rhyolite,  displays 
marked  banding.  The  foregoing  rocks  constitute  the  mountain  masses 
on  both  sides  of  the  canyon  to  the  highest  elevations,  the  gorge  cut 
through  them  being  sharp  and  rugged. 

Beneath  the  rhyolite  last  described  is  a  heavy  series  of  dark-gray 
quartzites  (Algonkian?),  which  continue  for  8  or  9  miles  along  the 
canyon  to  a  point  about  18  miles  above  Salmon  City.  Here  an  augite- 
hornblende-andesite  abruptly  cuts  the  schists,  the  rock  varying  from 
dark-gray  to  pink,  with  a  microcrystalliue  groundmass  and  porphyritic 
feldspar.  In  this  same  vicinity,  judging  from  fragments,  is  also  a 
quartz-bearing  porphyry,  possibly  related  to  rhyolite.  The  audesite 
extends  along  the  east  side  of  the  river  for  at  least  a  half  mile,  when 
it  passes  beneath  quartzite  debris  from  the  mountains  above.  On  the 
west  side  of  the  river,  a  mile  above  its  lower  edge,  it  is  overlain  for  a 
short  distance  by  60  to  75  feet  of  lava,  showing  columnar  structure. 
Between  21  and  22  miles  above  Salmon  City,  at  the  upper  end  of 
Edson's  ranch,  occurs  a  basic,  often  amygdaloidal  basalt,  almost  free 
from  feldspar,  and  carrying  a  green  or  red  alteration  mineral.  Toward 
the  lower  end  the  outcrop  pitches  gently  south,  but  changes  to  an 
equal  amount  to  the  north,  or  becomes  horizontal,  a  short  distance  on. 
There  are  two  varieties  of  the  rock,  one  hard  and  dark-gray,  the  other 
pink,  light,  and  vesicular.  It  appears  more  as  a  valley  rock  than  one 
entering  into  the  structure  of  the  higher  parts  of  the  mountains 
proper. 

About  28  miles  above  Salmon  City,  schists  again  appear  on  the  east 
side  of  the  river  for  2  or  2 £  miles,  to  be  then  again  cut  out  by  the  basalt, 
which  on  the  west  side  of  the  river  has  continued  all  along.  At  a  point 
31  miles  above  Salmon  City  another  series  of  rhyolitic  and  closely 
related  rocks  appears  from  beneatlrthe  basalt,  with  a  general  pitch  of 
20°  to  30°  northward,  or  downstream.  The  eruptive  first  beneath  the 
basalt  is  a  banded,  pink  trachyte.  This  is  apparently  300  to  400  feet 
thick.  Beneath  it,  with  conformable  pitch,  is  a  pink  rhyolite,  imme- 
diately succeeded  by  a  brilliant-green  volcanic  breccia.  The  matrix  of 
this  last  rock  consists  of  coarsely  crystalline  or  microcrystalline  rhyo- 
litic material;  the  fragmental  components  are  either  green  or  pink 
rhyolite,  or  possibly  trachyte,  an  associated  rock  in  this  region.  The 
fragments  are  small  to  large,  angular  or  slightly  rounded  by  attrition, 
and  the  pink  variety  of  rhyolite  or  trachyte  is  occasionally  found 
vesicular.  There  is,  altogether,  a  close  resemblance  between  the  frag- 
mental constituents  of  this  breccia  and  the  eruptives  occurring  in 


ELDRIDGE.J        ERUPTIVES    FROM    SALMON   CITY    TO    CHALLIS.  245 

association  with  it.  Stratification  is  occasionally  strongly  developed 
in  this  breccia,  notably  at  a  point  about  38  miles  above  Salmon  City. 
The  dip  is  to  the  northward,  the  strike  bending  between  northeast  and 
northwest. 

The  series  of  eruptives  thus  enumerated  closely  resembles,  in  field 
appearance  at  least,  that  observed  on  Silver  Creek,  20  or  30  miles  west. 
The  succession  of  the  various  types  may  differ  somewhat  in  the  two 
localities,  but  similar  composition,  structure,  and  general  occurrence 
are  clearly  perceptible. 

The  bright-green,  rhyolitic  breccia  extends  along  the  river,  with  but 
few  interruptions,  to  the  Pahsimeri  Valley,  a  distance  of  nearly  12 
miles,  forming  the  canyon  walls  to  a  height  of  500  or  600  feet  or  reced- 
ing to  more  distant  hills  according  to  the  extent  of  the  erosion.  It  is 
occasionally  cut  by  one  of  the  other  eruptives,  or  its  continuity  may  be 
interrupted  by  an  anticline  of  Cambrian  ( t)  quartzite,  common  in  the 
region  between  here  and  Chain's.  Three  or  4  miles  below  the  mouth  of 
the  Pahsimeri  the  breccias  are  capped  by  heavy  lava  flows,  probably  of 
hornblende-andesite,  a  rock  of  frequent  and  conspicuous  occurrence 
between  here  and  the  upper  end  of  the  Pahsimeri-Salmon  Valley,  3 
miles  above  the  mouth  of  the  former  stream.  In  the  latter  locality  the 
andesite  abuts  against  a  quartzite  anticline,  and  also  appears  on  the 
slopes  of  hills  to  the  east  at  much  higher  elevations.  West  of  the 
Salmon  an  old  valley  in  the  northern  slope  of  the  quartzite  ridge  is 
filled  with  eruptives,  appearing  at  a  distance  to  be  partly  breccia  and 
partly  audesite.  These  eruptives  again  appear  south  of  this  anticline, 
still  abutting  against  the  quartzites  or  capping  the  lower  spurs  of  the 
ridge.  From  here  they  continue  to  the  Challis  Valley,  forming  river 
bluffs  and  adjacent  hills.  They  surround  the  quartzite  anticlines  of 
this  region,  and,  locally,  rise  slightly  upon  their  sides.  At  the  lower 
end  of  the  Challis  Valley,  at  the  entrance  to  the  canyon  of  the  Salmon, 
the  east  bluff  of  the  river  shows  several  hundred  feet  of  interbedded 
breccia  and  andesite,  the  layers  often  presenting  uneven  lines  of  separa- 
tion, or  wedging  out  entirely,  all  without  semblance  of  regularity.  Just 
back  of  the  town  of  Challis  is  the  last  exposure  of  the  breccia  observed 
along  the  route. 

It  was  impossible  to  delineate  the  area  occupied  by  the  breccias; 
their  extent  along  the  Salmon  has  been  given — 35  or  40  miles.  East 
of  the  river  they  seem  to  be  limited  within  "a  few  miles  (2  or  3)  by  the 
quartzites  or  other  rocks  of  the  Salmon  Mountains,  excepting,  perhaps, 
along  the  Pahsimeri  River,  up  which  they  extend  for  at  least  4  or  5 
miles,  possibly  farther.  West  of  the  Salmon  the  extent  of  the  breccias, 
for  the  southern  portion  of  their  area  at  least,  is  considerably  greater, 
possibly  5  or  10  miles  from  the  river  in  some  instances.  The  nature 
and  configuration  of  the  deposit  as  a  whole  are  difficult  to  understand 
without  detailed  work.  Ehyolites,  andesites,  basalts,  and  breccias  are 
associated  at  one  point  or  another,  and,  as  in  the  region  lower  down 


246  GEOLOGICAL    RECONNAISSANCE    ACROSS   IDAHO. 

the  Salmon  and,  again,  about  the  Three  Forks  of  Carnas  Creek,  would 
seem  to  form  yet  another  great  volcanic  center. 

At  the  upper  end  of  the  Challis  Valley,  along  the  foot  of  the  range 
inclosing  it  on  the  south,  occur  enormous  bodies  of  a  gray  basic  audesite 
containing  both  hornblende  and  angite  and  grading  into  basaltic  varie- 
ties. Their  outcrop  on  the  river  is  several  hundred  feet  in  height  and 
between  2  and  3  miles  in  length,  and  they  apparently  extend  for  a 
considerable  distance  into  the  hills  on  either  side. 

About  6  miles  east  of  the  Salmon  Iliver  are  several  small  exposures 
of  greenish-gray  andesitic  or  rhyolitic  tuff  dipping  about  15°  northeast. 

The  divide  between  Warm  Spring  and  Antelope  creeks  is  capped 
with  a  thin  layer  of  bright-red  rhyolite.  The  groundmass  is  homo- 
geneous, and  the  porphyritic  mineral,  chiefly  feldspar,  at  a  minimum. 
The  rhyolite  overlies  green  and  yellow  tuffs,  similar  in  appearance  to 
those  seen  elsewhere  in  the  Challis  Valley.  The  entire  series  seems 
to  have  a  gentle  rise  from  periphery  to  center,  forming  a  dome  2  or  3 
miles  across. 

The  Challis  Valley  abounds  in  eruptives  of  one  kind  or  another  that 
may  be  seen  in  outcrops,  low  domes,  buttes,  ridges,  etc.,  scattered  over 
the  greater  part  of  its  area.  It  is  preeminently  an  area  of  volcanic 
rocks  modified  by  erosion  and  the  deposit  of  materials  derived  from 
the  inclosing  ranges,  sedimentary  and  crystalline. 

In  the  Wood  River  Valley,  both  in  the  vicinity  of  Ketchum  and  for 
several  miles  above  and  below,  are  numerous  dikes  or  other  irregular 
masses  of  audesite,  embracing  the  hornblende- mica  and  hornblende 
varieties,  with  feldspar  porphyritically  developed.  The  larger  bodies 
are  several  miles  in  extent,  showing  in  massive  outcrops  in  the  valley 
above  the  town,  and  below,  extending  well  into  the  ranges  west  of  the 
river.  On  Deer  Creek,  8  miles  below  Ketchum,  there  is  a  small  out 
crop  of  andesite,  but  its  continuity  with  bodies  higher  up  the  main 
stream  was  not  proved.  The  general  trend  of  the  larger  bodies  of 
eruptives  is  north-northeast ;  the  pitch,  westward.  The  eruptives  cut 
the  great  shale  and  limestone  series  forming  the  mass  of  the  mountains 
in  this  district.  They  seem,  however,  to  be  quite  independent  of  the 
ore  bodies  of  the  Wood  River  district,  and  therefore  to  have  had  little 
if  any  influence  upon  their  mineralization ;  but  the  entire  region  is  one 
of  marked  metamorphism  and  mineral  impregnation. 

A  few  miles  south  of  the  Wood  River  district  lie  the  plains  of  the 
Snake  River  and  its  tributaries  in  this  region.  Among  the  latter,  the 
valley  of  the  Big  Cainas  is  underlain  by  heavy  sheets  of  augite 
andesite,  which  also  extend  to  the  higher  lands  along  the  base  of  the 
mountains  both  north  and  south.  A  prominent  sheet  appears  along 
Camp  Creek,  extending  from  1  or  2  miles  below  Doniphan  to  the  mouth, 
resting  upon  granite,  which  is  exposed  at  one  or  two  points.  The  flow 
has  a  pitch  to  tlie  south,  as  though  acquired  by  the  general  inclination 
of  the  underlying  surface.  Bodies  of  this  eruptive  also  appear  to  the 


ELDRIDOE.J  ERUPTIVKS    OP    SILVER    CITY    DISTRICT.  247 

northeast  of  Donipbau.  The  ridge  south  of  Camas  Prairie,  separating  it 
from  the  Snake  Plains,  appears  from  a  distance  to  be  largely  composed 
of  augite-andesite,  granite,  by  report,  being  the  other  constituent.  On 
High  Prairie,  at  the  head  of  Big  Camas  Creek j  are  still  other  heavy 
outcrops  of  augite-andesite;  and  indeed  it  is  quite  likely  that  the  entire 
floor  between  the  granites  on  the  north  and  south  is  underlain  with  it, 
though  now  covered  with  wash  from  the  mountains. 

These  andesites  are  probably  directly  related  in  occurrence  and  com- 
position to  the  basalts  of  the  Snake  Plains  and  of  the  Boise  country  a 
short  distance  farther  west,  and  to  those  within  the  mountains  to  the 
north. 

In  the  granite  of  the  foothills  between  High  Prairie  and  the  Boise 
are  several  dikes  of  fine  and  coarsely  crystalline  quartz-porphyries. 
The  dikes  are  narrow  and  have  a  lineal  extent  of  not  over  1  or  2  miles. 
Similar  dikes  have  already  been  cited  on  the  western  slopes  of  the 
Boise  Kange  and  along  the  Boise  Canyon  within  the  range  itself. 

From  the  road  from  Nampa  to  Silver  City,  across  the  broad  plains  of 
the  Snake,  may  be  seen  numerous  exposures  of  basalt  in  sheets  hori- 
zontal, or  buckled  in  low  dome-like  elevations  from  a  half  mile  to  2  or 
3  miles  in  diameter  and  100  to  500  feet  high.  Of  the  latter  occurrence 
Initial  Point  is  an  illustration.  In  some  instances  erosion  has  begun 
to  take  effect  at  the  center  of  the  dome,  probably  from  a  greater  abun- 
dance of  cracks  than  elsewhere.  Along  the  Snake,  in  the  vicinity  of 
this  road,  the  lava  shows  in  precipitous  walls,  resting  upon  the  Ter- 
tiary beds  which  are  exposed  in  the  lower  portion  of  the  bluffs.  South 
of  the  river,  in  both  valley  and  mountains,  are  enormous  bodies  of  erup- 
tives  of  various  types,  in  dikes,  sheets,  and  irregular  masses.  In  one 
or  two  localities  certain  of  them  carry  opals  of  gem  value. 

The  eruptive  rocks  of  the  De  Lamar  and  Silver  City  district  embrace 
rhyolite,  diabase,  and  basalt.  The  country  rock  of  the  De  Lamar  mine 
is  rhyolite  of  rather  fine,  even  texture.  The  general  color  in  the  mines 
and  in  many  places  outside  is  white,  but  a  darker,  greenish  variety  also 
occurs.  The  extent  of  the  rhyolite  body  is  said  to  be  about  5  by  3 
miles,  the  greater  dimension  lying  northwest-southeast.  In  the  Black 
Jack  mine  the  country  rock  is  also  rhyolite,  very  similar  to  that  of  the 
De  Lamar  mine,  but  whether  of  the  same  body  was  not  ascertained. 
The  rhyolite  becomes  highly  kaolinized  in  a  certain  part  of  the  mine, 
and  is  then  mistaken  by  the  miners  for  granite,  and  is  so  called.  Dia- 
base, called  by  the  miners  "trachyte,"  also  occurs  as  a  dike  in  the  Black 
Jack  mines,  and  basalt  appears  as  a  surface  flow  a  half  mile  below  the 
mine  on  the  mountain  side.  The  occurrence  of  these  eruptives  and 
their  relations  to  one  another  are  somewhat  obscure,  except  that  the 
diabase  and  basalt  are  later  than  the  rhyolite. 

The  effect  of  the  eruptives  upon  the  mineralization  of  the  ore  bodies 
will  require  detailed  observations  in  field  and  chemical  laboratory. 
The  rhyolite  uear  the  veins  nearly  always  shows  some  degree  of 
mineralization. 


248  GEOLOGICAL   RECONNAISSANCE    ACROSS    IDAHO. 

GENERAL,  STRUCTURAL,  FEATURES. 

The  mountains  of  the  southern  half  of  Idaho  are  developed  en  masse 
or  in  ranges  the  lines  of  which  have  a  north:northwest  or  an  east- 
northeast  direction.  The  development  en  masse  is  confined  to  the 
western  half  of  the  State;  the  system  of  ranges,  although  not  confined 
to  the  eastern,  is  nevertheless  its  chief  order  of  structure.  In  the  moun- 
tains en  masse  it  is  quite  probable  that  the  elevations  were  attained  in 
pre-Paleozoic  times,  while  the  range  system  may  have  undergone  several 
periods  of  development,  to  late  Tertiary. 

The  range  of  primary  importance  is  the  Continental  Divide.  This 
pursues  a  somewhat  irregular  trend  along  the  State's  border,  but  east 
of  the  Lemhi  and  Salmon  valleys  for  a  distance  of  nearly  100  miles  it 
has  a  general  course  between  N.  30°  and  50°  W.,  which  is  also  that  of 
its  structural  axis.  The  quartzites  and  schists  showing  in  its  western 
face  are  locally  crumpled,  but  the  prevailing  dip  is  50°  or  60°  SW. 
Regarding  the  nature  of  the  uplift,  it  is  believed  to  be  an  anticline,  but 
time  permitted  an  examination  at  only  one  point  on  the  western  side. 
.  The  development  of  the  Continental  Divide,  and  also  of  the  ranges  to 
the  west,  has  doubtless  continued  in  varying  degree  from  early  times  to 
the  Miocene,  for  nonconformities  occur  within  the  crystalline  series,  and 
in  the  Tertiary  beds  of  the  Salmon  City  Valley  folding  of  considerable 
importance  may  be  seen,  along  both  the  Lemhi  and  Salmon  rivers,  and 
east  of  them,  in  the  region  of  Kirtley  and  Carmen  creeks. 

Parallel  with  the  fold  of  the  Continental  Divide  in  Idaho  are  numer- 
ous others,  represented  in  the  Lost  and  Salmon  River  ranges  and 
those  immediately  to  the  west.  The  greater  part  of  these  ranges  lay 
so  far  beyond  the  route  of  survey  that  their  structure  could  be  seen 
only  remotely.  Their  northwestern  ends,  however,  were  in  many 
instances  observed  along  the  Salmon  and  Lost  rivers  in  the  passage 
from  Salmon  City  to  Wood  River. 

The  general  strike  here  encountered  was  about  N.  35°  W.,  the  dip 
now  northeast,  now  southwest.  This  is  shown  in  the  areas  of  Algon- 
kian  (?)  schists  and  quartzites  between  Salmon  City  and  the  Pahsimeri, 
and  in  the  several  anticlines  that  occur  in  what  have  been  provision- 
ally regarded  as  Cambrian  quartzites,  extending  along  the  Salmon 
River  from  a  point  10  or  12  miles  below  the  Pahsimeri  to  Challis. 
Many  of  the  anticlines,  having  been  cut  by  the  river  near  one  end  or 
the  other,  show  both  exterior  and  interior  structure,  presenting  canyon 
walls  sometimes  in  sheer  precipices  1,000  to  1,500  feet  high.  While  the 
majority  of  the  folds  passed  are  anticlines,  others  are  quite  possibly 
monoclines,  the  result  of  faulting,  though  this  was  suggested  rather 
than  proved  by  minor  strike  faults  and  by  the  general  configuration  of 
some  of  the  ridges  observed. 

In  the  mountains  of  the  Wood  River  region  the  development  is  again 
upon  a  N.  30°  W.  trend;  the  structure  is  anticlinal,  the  axis  of  elevation 
passing  east  of  the  river,  in  the  mountains  between  this  and  the  Lost 


ELDRIDOK.]  STRUCTURE.  249 

and  Salmon  rivers.  The  northern  extent  of  this  anticline  is  unknown; 
to  the  south,  as  observed  from  a  distance,  it  apparently  sinks  beneath 
the  Snake  Plains.  Between  the  Wood  River  ranges  and  the  main 
Sawtooth,  about  the  head  of  Salmon  River,  is  a  complexity  of  folds, 
the  details  of  which  could  not  be  deciphered  from  a,  distance. 

In  the  region  thus  far  described  the  folds  which  have  governed  the 
range  structure  are  particularly  clear  by  reason  of  the  presence  of  the 
sedimentaries,  altered  or  unaltered.  In  the  region  of  granites  to  the 
west,  however,  structural  lines  are  more  indefinite,  but  northwest- 
southeast  trends,  with  dips  to  southwest  or  northeast,  may  still  be 
found  in  sufficient  number  to  warrant  the  belief  that  certain  of  the 
ranges,  at  least,  were  developed  on  the  same  lines  as  those,  more  pro- 
nounced, in  the  eastern  portion  of  the  State.  Among  these  are  the 
Sawtooth  and  Boise  ranges,  although  the  former  has  been  greatly 
influenced  by  the  texture  of  the  rocks  entering  into  its  composition. 
Still,  even  the  lines  due  to  this  influence  were  probably  governed  origi- 
nally by  the  direction  assumed  in  the  flssuring,  which  controlled  the 
trend  of  the  rocks  of  eruptive  origin.  This  is  particularly  noticeable 
along  the  crest  of  the  Sawtooth. 

An  east-northeast  structure  is  suggested  at  many  points  in  the  gran- 
ites of  the  western  half  of  the  State.  It  is  often  but  incipient,  shown 
chiefly  in  lines  of  jointing,  in  occasional  strikes  of  foliation  planes,  or 
in  the  trend  of  fissures,  but  the  constant  recurrence  of  these  features 
indicates  its  prevalence,  and  that  the  forces  which  produced  it  acted 
with  greater  or  less  energy  over  a  large  portion  of  the  western  half  of 
southern  Idaho.  Many  local  instances  of  the  lighter  effects  occur  about 
Rocky  Bar  and  Atlanta,  but  the  most  pronounced  illustration  is  the 
transverse  ridge  which  forms  the  divide  between  the  main  Salmon  and 
its  Middle  Fork.  This  ridge  is  an  east-northeast  anticline,  the  strata, 
whether  granite  or  schist,  quartzites  or  limestones,  all  showing  the  dip 
away  from  the  center.  The  structure  is  traceable,  with  some  obscure 
intervals,  to  the  drainage  of  Camas  Creek,  within  a  few  miles  of  the 
Yellow  Jacket  mining  district.  The  interruptions  are  due  both  to  com- 
plex folding  and  to  the  presence  of  large  bodies  of  eruptives. 

The  divide  between  the  waters  of  the  Salmon  and  Snake  rivers  can 
not  properly  be  considered  a  range  in  a  strictly  structural  sense,  for  it 
is  evidently  composite  in  character.  For  40  or  50  miles  east  of  the  Saw 
tooth  it  is  probably  a  complex  of  folds.  East  of  this  it  is  formed  by  the 
transverse  links  between  the  several  ranges  of  northwest  trend,  or  by 
the  ranges  themselves,  and  to  this  combination  is  due  the  extraordinarily 
irregular  trend  of  one  of  the  most  important  watersheds  of  the  State. 

West  of  the  Sawtooth  Range,  with  one  or  two  exceptions,  the  loci  of 
the  divides  between  the  drainage  basins  are  doubtless  partly  due  t« 
early  accidents  of  structure — folding,  faulting,  jointing — but  in  the 
wide  distribution  of  the  homogeneous  granite  the  structural  features 
are  only  slightly  pronounced  and  erosion  has  exerted  its  full  influence 


250  GEOLOGICAL    RECONNAISSANCE    ACROSS    IDAHO. 

in  modeling  the  configuration  of  to-day.  The  Boise  Range,  however, 
is  clearly  developed  on  northwest-southeast  lines  of  structure,  a  part 
of  the  general  system  of  the  State. 

The  intermontane  valley  at  Horse  Shoe  Bend,  Payette  River,  like  that 
at  Salmon  City,  shows  several  flexures  in  its  Tertiary  beds.  The  most 
important  is  an  anticline  running  diagonally  across  the  center  of  the 
valley  southeast  to  northwest,  the  other  folds  being  subordinate  to  this, 
but  developed  in  the  same  lines. 

The  ridge  between  the  valley  of  Big  Camas  Creek  and  the  Snake, 
which  has  a  nearly  east  and  west  trend  for  50  or  60  miles,  shows  from 
a  distance  a  number  of  broad,  gentle  flexures,  but  their  axial  lines  were 
not  clearly  visible  from  the  route  of  travel.  The  western  half  of  the 
ridge,  at  the  head  of  Little  Camas  Creek,  is  a  part  of  the  general  sys- 
tem of  elevations  which  belongs  to  the  mountain  region  just  north. 

The  Owyhee  Kange  in  southwestern  Idaho  belongs  to  the  type  of 
Desert  Ranges.  It  is  of  granite,  cut  by  eruptives,  with  Pleistocene 
and  probably  Tertiary  sediments  occupying  the  small  interior  valley 
of  Reynolds  Creek.  The  sedimentary  beds  show  slight  folding,  indi- 
cating movement  in  later  times,  but  the  range  received  its  greatest 
development  prior  to  the  Tertiary. 

MINING    DISTRICTS. 

GOLD   AND    SILVER. 

The  mining  districts  of  the  precious  metals  visited  in  the  reconnais 
sance  include  the  region  about  Rocky  Bar  and  the  adjacent  Red  War- 
rior camp,  together  known  as  the  Bear  Creek  district;  the  celebrated 
Atlanta  lode  and  its  attendant  veins;  the  Yellow  Jacket  camp;  the 
California  Bar,  Leesburg,  and  Lemhi  placers;  Wood  River;  and  the 
Silver  City  and  De  Lamar  camps  in  the  Owyhee  Range.  All  are  gold 
or  silver  producers  in  varied  degrees,  and  have  histories  dating  back 
from  five  to  thirty  years,  with  yields  reported  in  some  instances  as 
enormous.  At  the  present  time  the  industry  throughout  the  State  is, 
with  a  few  conspicuous  exceptions,  at  a  low  ebb. 

THE   BEAR   CREEK   DISTRICT. 

Under  this  are  included  the  Rocky  Bar  and  Red  Warrior  camps  and 
the  region  immediately  about — in  all,  an  area  3  or  4  miles  in  diam- 
eter. Placers  formerly  extended  along  the  main  stream  to  points  con 
siderably  beyond,  but  are  now  worked  by  a  few  Chinese  only.  The 
town  of  Rocky  Bar  lies  well  within  the  southern  limit  of  the  great 
mountain  mass  of  Idaho,  in  a  deep,  narrow  gulch  near  the  junction  of 
Beaver,  Steel,  Feather,  and  Bear  creeks,  from  the  latter  of  which  the 
district  receives  its  name.  The  elevation  at  Rocky  Bar  is  4,800  feet; 
the  surrounding  ridges  rise  to  6,500  feet;  the  crest  of  the  divide  between 
this  and  the  Atlanta  district,  to  8,500  feet;  while  Mount  Steel,  the 
highest  peak  of  the  region,  attains  an  altitude  of  9,500  feet.  Commu- 
nication is  by  a  good  wagon  road  to  Mountain  Home  Station,  on  the 


EI.UKIDOE.] 


BEAR   CREEK    DISTRICT. 


251 


Oregon  Short  Line,  Cl  miles  distant.  Eed  Warrior  Camp  lies  from 
1  to  2  miles  southwest  of  Bocky  liar,  about  the  heads  of  Eed  Warrior 
Creek,  a  short  tributary  of  Bear  Creek.  The  dividing  ridge  between 


115  ao 


us  ao 


Fio.  39 Sketch-map  of  Bear  Creek  district. 


3MIUE& 


the  two  camps  is  500  to  1,000  feet  higher  than  Bear  Creek,  and  has  a 
general  trend  northwest.  A  wagon  road  leads  from  the  camp  to  the 
main  thoroughfare  from  Eocky  Bar,  following  the  stream. 


252  GEOLOGICAL   RECONNAISSANCE    ACROSS    IDAHO. 

The  country  rock  of  the  district  is  the  gray  granite  entering  so  largely 
into  the  constitution  of  the  Boise  and  Sawtooth  ranges.  It  is  an  aggre- 
gate of  feldspar,  quartz,  and  mica,  chiefly  biotite.  The  feldspar  occa- 
sionally becomes  porphyritic.  The  granite  is  inclined  to  the  massive 
form,  but  it  is  not  altogether  devoid  of  foliation,  which  iu  some  instances 
strikingly  suggests  bedding.  The  prevailing  strike  of  the  foliation 
planes  is  N.  50°  to  70°  E.,  while  their  dip  is  generally  toward  the  north- 
west. Minor  flexures,  however,  occur  and  the  entire  country  displays 
more  or  less  fracturing,  especially  along  the  gulches. 

Eruptives  occur  as  narrow  dikes  in  the  granites.  They  include  basalt 
or  olivine-diabase,  quartz- porphyry  similar  to  the  Leadville  porphyry, 
and  diorite-porphyrite.  These  eruptives  were  found  at  many  poiuts 
along  the  route  of  survey  in  localities  not  noted  for  the  presence  of  ore 
bodies,  and  their  occurrence  in  a  region  of  metalliferous  veins,  notwith- 
standing they  contain  locally  a  greater  or  less  amount  of  finely  dissem- 
inated sulphide  of  iron,  may  have  been  quite  without  influence  upon 
the  mineralization  of  the  district;  indeed,  their  mineral  contents  may 
have  been  derived  from  the  same  source  as  those  of  the  ore  bodies. 
The  quartz-porphyry  of  Leadville  type  which  occurs  in  a  tunnel  in  Red 
Warrior  Gulch  a  short  distance  above  the  camp,  and  perhaps  at  other 
points  as  well,  is  particularly  rich  in  pyrite. 

The  mineral  veins  throughout  the  Bear  Creek  district  are  of  quartz, 
carrying  auriferous  sulphides  below  water-level  and  their  alteration 
products  and  free  gold  above.  Most  of  the  veins  strike  and  dip  with 
the  foliation  planes;  occasionally,  however,  they  may  be  found  crossing 
these.  In  width  they  occur  up  to  12  feet,  or  iu  some  places  there  may 
be  a  number  of  small  veins,  separated  from  one  another  by  narrow 
bands  of  granite,  the  whole  forming  a  single  mineral-bearing  zone  5  to 
10  or  15  feet  wide. 

The  included  granite,  and  also  that  for  4  or  5  feet  on  either  side 
whether  of  zone  or  single  vein,  usually  shows  a  marked  change  in 
appearance.  The  micaceous  compounds,  particularly  the  biotite,  have 
suffered  greater  or  less  decomposition,  with  dissemination  of  the  iron 
throughout  the  rock  mass,  and  the  feldspars  have  become  kaolinized 
in  considerable  degree.  The  granite -appears,  in  fact,  as  a  crystalline 
aggregate  of  feldspar  and  quartz,  more  or  less  ferruginous  and  dis- 
integrated. This  is  commonly  designated  "vein  matter."  Such  a  zone 
of  alteration  is  visible  not  only  in  open  cuts  and  mines  but  frequently 
with  great  distinctness  along  the  outcrop  of  a  vein. 

The  quartz  is  coarse  to  finely  crystalline,  occasionally  banded,  and 
carries  the  mineral  either  disseminated,  or  in  bunches,  or  following  the 
planes  of  structure  in  thin  seams.  The  material  varies  in  richness  of 
impregnation  and  the  sulphides  themselves  in  gold  contents.  The 
prevailing  sulphide  is  pyrite,  but  those  of  antimony,  zinc,  galena,  and 
copper  are  also  present.  The  last  is  rare,  and  the  zinc  and  galena 
sulphides  are  of  minor  importance. 


ELDKIDQE.]  ATLANTA    DISTRICT.  253 

The  gold  of  the  district  is  light  colored  and  is  said  to  be  worth 
about  $14  per  ounce,  the  ore  running-  from  $6  to  $30  per  ton  without 
concentration. 

The  openings  in  the  lied  Warrior  camp,  with  the  exception  of  the 
Wide  West,  which  has  been  closed  during  the  past  season,  are  hardly 
more  than  prospects,  developed  by  tunnels.  The  veins  usually  follow  the 
planes  of  foliation,  the  prevailing  strike  being  between  northeast  and 
east,  varying  slightly.  At  one  or  two  points  a  crushed  condition  of  the 
granite  was  observed,  with  what  appeared  to  be  numerous  small  aud 
irregular  gash  veins.  A  suspected  dip  of  the  fractured  zone  was 
80°  NE.' 

The  mines  about  Rocky  Bar  embrace  some  of  historic  interest — the 
Old  Alturas.  Idaho,  and  Vishnu — while  the  placers  in  early  days  were 
almost  phenomenal  in  their  product.  At  present  the  industry  is  quiet, 
aud  although  a  depth  of  700  feet  has  been  attained  in  some  of  the 
mines,  only  the  upper  levels  could  be  examined.  Quartz  veins  occur 
singly  and  in  zones ;  in  the  former  case,  of  widths  up  to  5  or  6  feet, 
locally  even  greater;  in  the  latter,  including  intervening  granite,  up 
to  20  feet,  the  entire  zone  being  mined.  The  latter  condition  prevailed, 
it  is  said,  in  the  Old  Alturas  mine.  The  trends  of  the  veins  in  this 
camp  vary,  but  one  between  northeast  and  southeast  prevails ;  the  dip 
is  usually  south,  though  in  one  or  two  instances  north.  The  upper 
portions  of  the  veins  are  sometimes  broken  over  in  one  direction  or  the 
other,  dipping  as  low  as  30°;  in  the  lower  levels  the  dip  is  said  to  be 
nearer  vertical. 

THE   ATLANTA    DISTRICT. 

This  district  lies  18  miles  northeast  of  Eocky  Bar,  across  the  divide 
between  the  South  and  Middle  forks  of  the  Boise  River,  at  the  junction 
of  the  latter  stream  with  the  Yuba.  The  center  of  mining  interest  is 
Atlanta  Hill,  an  eminence  of  1,500  or  2,000  feet  in  the  forks  of  the 
streams,  the  end  of  a  spur  from  the  Sawtooth  Range  to  the  east.  The 
town  of  Atlanta  lies  in  the  valley  of  the  Middle  Fork,  at  the  northern 
base  of  the  hill,  5,500  feet  above  sea-level.  From  this  side  three  gulches 
enter  the  valley,  Moutezuma  to  the  east,  Quartz  in  the  center,  and  a 
short,  nameless  one  to  the  west.  The  first  two  head  quite  at  the  crest 
of  the  hill.  The  southern  face  of  the  hill  is  not  so  deeply  eroded,  and 
at  its  base  Grouse  Creek  flows  in  nearly  direct  line  to  the  Yuba.  Con- 
fined to  the  hill,  so  far  as  at  present  known,  is  the  celebrated  Atlanta 
lode  and  its  associated  fissures.  At  the  east  both  hill  and  lode  seem 
to  end  abruptly  against  a  higher  portion  of  the  spur,  while  to  the  west, 
beyond  the  Yuba,  the  presence  of  the  lode  has  never  been  definitely 
established.  South  of  Atlanta  Hill,  beyond  Grouse  Creek,  the  moun- 
tains, still  spurs  of  the  Sawtooth,  rise  to  heights  of  9,000  feet,  form 
ing  the  divide  between  the  Yuba  and  South  Fork  of  the  Boise.  North 
of  Atlanta,  across  the  Middle  Fork,  uniting  Greylock  (9,000  feet) 
and  another  lower  peak  to  the  west,  is  a  prominent  ridge  capped  with 


254       GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

morainal  or  other  drift,  the  crest  fully  2,000  feet  above  the  present  level 
of  the  river.  The  Atlanta  Valley  is  a  gravel  Hat  3  or  4  miles  long  by 
1  or  2  wide.  The  mountains  about  are  sharp  and  rugged,  the  canyons 
often  impassable,  with  evidence  of  glacial  action  in  nearly  all  their 
upper  portions.  The  district  is  50  or  60  miles  within  the  southern 
limits  of  the  great  mountain  mass  of  Idaho,  and  but  for  a  good  wagon 
road  to  Rocky  Bar  and  thence  to  Mountain  Home  Station  on  the  Oregon 
Short  Line,  would  be  difficult  of  access.  Trails  lead  from  Atlanta  to 
mining  camps  about,  among  them  Sawtooth  City  and  Vienna,  in  the 
upper  Salmon  Valley.  The  latter  places  are  connected  by  wagon  road 
with  the  Wood  River  region,  about  40  miles  distant. 

The  country  rock  of  the  Atlanta  district  is  gray  granite.  It  is  simi- 
lar in  composition  to  that  of  the  Bear  Creek  district,  but  structural 
planes  or  planes  of  foliation  are  not  so  prominent  as  there.  To  the  west 
and  east,  however,  gneissoid  granite  does  occur,  and  on  the  eastern 
side  of  the  Sawtooth  Range  are  some  very  fine  examples  of  this  type 
of  rock.  The  rock  of  the  Atlanta  district  affords  many  evidences  of 
extensive  fracturing,  and  it  may  be  that  to  this  is  due  the  difficulty  of 
recognizing  planes  of  foliation. 

The  eruptive  rocks  of  the  region  occur  in  dikes  of  greater  or  less 
prominence.  In  the  range  west  of  the  Yuba  and  Boise  rivers,  and 
also  in  the  high  point  in  the  elbow  of  the  latter  stream,  are  a  number 
of  dikes  100  to  200  feet  wide,  of  fine  to  coarsely  crystalline,  pink  quartz- 
porphyry  and  quartz-bearing  syenite-porphyry.  These  extend  several 
miles  across  country,  their  general  trend  being  a  little  north  of  west, 
their  dip  45°  or  50°  NNE.  to  vertical.  Aphanitic  syenite  (lampro- 
phyre)  also  occurs  in  a  small  east-aud-west  dike  in  Montezuma  Gulch, 
halfway  up,  and  again  in  the  Monarch  mine  in  Quartz  Gulch.  Other 
dikes  of  the  same  or  different  nature  are  reported  cutting  the  Atlanta  . 
lode  in  the  various  mines,  but  they  were  all  inaccessible.  In  the  Tahoma 
mine,  which  is  on  one  of  the  lateral  fissures,  a  dike  of  white  decom- 
posed porphyry,  25  to  50  feet  thick,  cuts  the  vein  at  a  very  acute  angle. 
Its  trend  is  X.  26°  W. ;  its  dip,  W.  45°  to  perpendicular.  On  some  of 
the  levels  it  has  thrown  the  vein,  but  this  is  again  brought  into  line 
within  50  or  60  feet  by  a  second  fault,  approximately  parallel  with  the 
dike,  marked  by  a  clay  course. 

The  influence  of  the  several  dikes  of  the  district  upon  the  minerali- 
zation of  the  veins  has  never  been  ascertained  by  those  who  have 
had  charge  of  the  mines,  and  the  time  allotted  to  the  reconnaissance, 
together  with  the  inaccessibility  of  many  of  the  mines,  did  not  permit 
the  examination  requisite  for  the  solution  of  such  a  problem.  The  dikes 
are  evidently  outflows  of  several  periods,  and  assumed  many  directions 
of  trend. 

The  central  feature  in  the  Atlanta  district  from  the  mining  stand- 
point is  a  main  lode  having  what  appear  to  be  numerous  nearly  parallel 
branches,  spurs,  or  feeders,  forming  acute  angles  with  it;  at  the  ends 


U.   8.   GEOLOGICAL  SURVEY 


SIXTEENTH  ANNUAL  REPORT      PART  II      PL.  XVI 


"        ^  /r'  " 


SCALE 


SKETCH   MAP  OF  ATLANTA  DISTRICT. 


ELDBIDOE.]  ATLANTA    DISTRICT.  255 

the  lode  itself  may  split.  The  main  lode  is  known  as  the  Atlanta;  the 
spurs  are  designated  by  the  names  of  the  several  mines  located  upon 
them.  The  outcrop  of  the  Atlanta  lode  has  an  almost  uniform  trend  of 
N.  50°  to  60°  E.,  and  occupies  a  position  on  or  near  the  summit  of 
Atlanta  Hill,  running  nearly  with  the  ridge.  Its  eastern  end  is  sup- 
posed to  be  near  Montezurna  Gap;  it  follows  thence  westward  along 
the  crest  of  the  ridge  to  the  drainage  of  Quartz  Gulch,  which  it  crosses 
about  one-fourth  to  one-third  mile  below  the  summit,  and  again  taking 
the  ridge,  continues  on  the  crest  to  the  western  slope,  down  which  it 
passes  to  Yuba  River.  The  total  length  of  the  lode  exposed  and  pros- 
pected is  2£  miles;  its  continuation  beyond  this  has  not  been  proved. 
The  width  of  the  lode  is  said  by  those  conversant  with  its  early  exploi- 
tation to  vary  between  50  and  150  feet,  75  being  regarded  a  fair  aver- 
age for  the  whole.  This  width  was  the  maximum  observed  by  the 
writer,  but  only  a  very  small  portion  was  accessible  to  him.  The  lode 
is  at  present  divided  between  four  or  five  companies :  one,  of  which 
Gen.  W.  H.  Pettit  is  the  manager,  at  the  eastern  end,  owning  1,500  or 
more  feet;  a  second,  controlling  a  few  feet  only,  coming  next;  the  third, 
the  Monarch,  extending  to  Quartz  Gulch;  a  fourth,  the  Buffalo,  to  the 
west  of  this;  and  the  fifth,  now  the  Atlanta  Consolidated  Gold  and 
Silver  Mining  Company,  occupying  several  thousand  feet  at  the  western 
end.  The  dip  of  the  Atlanta  lode  varies  in  direction.  At  the  eastern 
end  it  is  to  the  N.  70°  or  80°,  becoming  vertical  at  the  western  limit  of 
the  Pettit  mine;  in  the  Monarch  mine  a  change  to  the  south  takes  place; 
in  the  Buffalo  the  dip  continues  8.  45°  to  70° ;  while  for  the  western 
fourth  of  the  lode,  the  position  of  the  vein  is  again  vertical  or  with  but 
a  slight  southerly  underlay. 

The  vein  matter  of  the  Atlanta  lode  is  a  more  or  less  fragmental, 
clear  to  bluish-white  quartz,  and  an  altered  granitic  rock  in  which 
quartz  predominates  and  the  feldspars  are  highly  kaolinized  and  the 
micas  have  generally  disappeared.  This  second  rock  is  doubtless 
related  to  the  country  granite,  and  has  undergone  the  changes  so  com- 
mon in  rock  adjacent  to  or  in  mineral  veins.  More  or  less  clay  is 
present  locally,  and  some  calc-spar  in  lenticular  bodies.  The  entire 
mass  of  the  vein  is  very  friable  and  easily  mined.  The  quartz  is  the 
chief  metal-bearing  constituent,  and  occurs  from  a  thin  seam  to  one  6 
feet  across,  in  one  or  several  courses,  on  either  wall  or  in  the  interior. 
Although  a  considerable  proportion  of  the  vein  may  contain  ore  in  pay 
amount,  it  is  said  to  be  rare,  if  ever,  that  the  entire  width  is  mineralized 
to  this  extent.  Again,  it  is  often  the  case  that  narrower  streaks  con- 
tain as  large  amounts  of  gold  and  silver  as  the  broader.  The  granitic 
ledge  matter  shows  mineralization,  but  is  usually  regarded  as  too  low 
in  content  to  pay  for  mining.  The  calc  spar  is  considered  by  some 
miners  as  a  good  indication  of  proximity  to  a  rich  ore  body. 

The  ore,  according  to  General  Pettit,  who  has  been  familiar  with 
the  lode  from  its  earliest  exploitation,  lies  in  shoots,  four  or  five  in 


256      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

number,  along  the  length  of  the  vein;  one  in  the  mine  at  the  eastern 
end  of  the  lode;  one  in  the  Monarch,  pitching  westward,  toward  the 
dike  occurring  near  its  shaft;  one  just  beyond  this  dike,  also  pitching 
toward  it,  or  eastward  Cthis  in  the  Buffalo  mine) ;  and  one  or  two  in 
that  portion  of  the  ledge  now  being  opened  by  the  Atlanta  Consoli- 
dated Gold  and  Silver  Mining  Company.  Moreover,  in  these  shoots, 
the  ore  is  said  to  be  in  the  form  of  larger  or  smaller  lenticular  masses, 
which  overlap  one  another  in  depth,  occupying  different  positions  in 
the  width  of  the  vein. 

The  Atlanta  lode  has  the  appearance  of  an  original  fissure  the  filling 
of  which  has  been  crushed  •  by  reason  of  movements  belonging  to  a 
second  period  of  disturbance.  Mineralization  may  have  been  effected 
either  at  the  time  the  original  fissure  was  filled  or  subsequent  to  the 
fracturing  that  resulted  from  the  second  movement.  The  walls  of  this 
lode  are  horizontally,  diagonally,  or  vertically  slickensided;  there  is 
often  present  a  strong  clay  selvage — almost  always  a  trace  of  it;  and 
the  vein  matter  has  been  rendered  friable  by  a  marked  degree  of  frac- 
turing, the  sharp,  angular  fragments  often  forming  a  brecciated  mass, 
held  together  in  a  clay  of  tine  siliceous  cement.  Slickensides  and  clays 
are  also  present  in  the  interior  of  the  vein. 

The  mineral  contents  of  the  vein  are  free  gold  and  an  auriferous 
sulphide  of  iron — pyrite  or  the  related  arsenical  and  antiuionial  coin- 
pouuds.  Prof.  J.  E.  Clayton,1  from  an  early  and  more  complete  exami- 
nation of  the  lode — chiefly,  however,  in  the  Monarch  and  Buffalo 
mines — reports  the  metallic  contents  as  "  gold,  native  silver,  ruby 
silver,  brittle  silver  ore,  and  sulphide  of  silver  and  pyrite.  The  brittle 
silver,  or  black  antimouial  silver,  is  the  most  abundant  ore.  Next  in 
quantity  and  value  is  the  ruby  silver.  The  native  silver  and  silver 
glance  are  found  only  in  small  quantities.  The  free  gold  constitutes 
from  20  to  40  per  cent  of  the  total  value.  The  other  minerals  are  iron 
pyrites  in  moderate  quantities  disseminated  through  the  granular, 
friable  quartz  and  the  granitic  inclosures  of  the  lode.  I  saw  but  few 
traces  of  copper,  zinc,  or  lead."  Professor  Clayton  also  adds,  as  the 
result  of  his  observations,  that  "  much  of  the  quartz  in  this  lode  is 
comparatively  barren.  The  rich  streak  of  black  sulphuret  and  ruby 
ore  [probably  in  the  Monarch  and  Buffalo  mines]  varies  in  width  from 
1  foot  to  6  or  7  feet,  and  alongside  of  it  is  a  zone  of  pay  rock,  equally 
as  wide,  that  carries  a  good  percentage  of  free  gold  with  silver  ore 
disseminated  through  it,  making  the  pay  streak  from  2  to  15  feet  wide 
and  extending  in  length  underground,  in  the  Monarch  and  Buffalo 
claims,  nearly  2,000  feet  on  the  course  of  the  lode."  The  character  of 
the  ore  is  said,  however,  to  show  material  changes  in  the  length  of 
the  lode. 

As  worked  at  the  preseut  day  the  average  yield  of  the  lode  is  about 
$20  per  ton  in  gold.  From  this  it  varies  in  either  direction.  Ten 

1  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  V,  p.  471,  1876-77. 


ELDRIDOE.]  ATLANTA   DISTRICT.  257 

dollars  is  regarded  as  the  limit  of  profitable  working,  while  bunches  of 
ore  of  a  value  of  several  hundred  dollars  per  ton  are  occasionally  met 
with.  The  ore  is  now  hand-sorted,  a  portion  being  milled  in  the  camp, 
a  portion  concentrated  and  shipped  out.  In  the  early  days,  however, 
no  effort  was  made  to  save  ore,  even  the  richest,  that  would  not  mill 
free  gold;  many  of  the  dumps  at  the  present  time  are  therefore  corre 
spondingly  profitable,  and  every  season  a  portion  of  them  is  reworked 
by  hand  jigging  or  some  other  simple  process. 

The  average  proportion,  in  values,  of  gold  to  silver  in  the  ores  of  the 
main  lode  is  as  1  to  2;  in  the  lateral  veins  north  of  the  main  lode,  also 
as  1  to  '2;  but  in  some  at  least  of  those  south  the  proportion  is  said  to 
be  as  3  to  1. 

The  fissures  in  Atlanta  Hill  transverse  to  the  main  lode  have  an 
E.W.  to  N.  65°  W.  trend,  and  dip  north,  south,  or  vertical,  but  always 
steep.  Their  structural  relations  with  the  lode  are  undetermined;  some 
of  them  have  the  appearance  of  spurs,  offshoots,  or  "feeders,"  while 
others  may  prove  to  be  independent  veins.  They  lie  both  to  the  north 
and  south  of  the  main  lode,  and  those  on  opposite  sides  are  in  some 
instances  so  nearly  continuous  in  their  course  that  they  may  readily  be 
suspected  of  being  one  and  the  same  fissure.  In  no  instance,  however, 
has  the  passage  of  a  lateral  vein  into  the  main  lode  been  traced,  except 
perhaps  in  the  Pomeroy-Last  Chance.  Here  General  Pettit  states  that, 
with  other  objects  in  view,  he  traced  the  lateral  directly  into  the  main 
fissure,  without  a  separating  wall  of  any  kind,  the  ground  in  the  acute 
angle  of  the  two  veins  being  highly  fractured. 

The  lateral  veins  rarely  attain  a  thickness  greater  than  8  or  10  feet, 
and  often  but  1J  to  2  feet,  or  even  less.  Some  of  them  are  very  rich  in 
metallic  contents,  and  carry  mines  of  much  importance.  Among  these 
are  the  Last  Chance,  Big  Lode,  and  Tahoma,  the  first  being  to  the  south 
of  the  main  lode,  the  others  to  its  north.  The  ore  of  the  lateral  veins 
in  a  general  way  is  similar  to  that  of  the  main  lode,  consisting  of  free 
gold,  native  silver,  and  the  sulphides  already  mentioned,  in  a  quartz 
vein-stuff.  Not  all  of  the  spurs  show  a  fragmental,  breccia-like  charac- 
ter in  the  filling  to  the  extent  seen  in  the  mines  of  the  Atlanta  vein,  but 
the  feature  is  not  absent.  The  value  of  the  ore  of  the  transverse  veins 
is  said  to  vary  considerably  in  the  different  mines,  certain  of  them  pre- 
senting material  of  low  grade  but  of  steady  occurrence;  others,  mate- 
rial of  high  grade  in  smaller  and  more  intermittent  bodies.  The  rela- 
tive proportions  between  the  metallic  contents  also  vary  in  the  different 
veins,  not  only  on  the  same  side  of  the  main  fissure,  but  on  opposite 
sides. 

The  Atlanta  lode  and  its  branches  have  yielded  in  past  years  enor- 
mous sums,  and  a  favorable  inference  may  well  be  drawn  from  history 
and  observation  as  to  the  possibilities  in  the  future.  The  lode  is  well 
located  with  regard  to  economic  development,  while  timber  and  mining 
supplies  may  be  obtained  with  comparative  ease. 
16  GKEOL,  PX  2 17 


258  GEOLOGICAL   RECONNAISSANCE   ACROSS   IDAHO. 

THE    SHEEP   MOUNTAIN   DISTRICT. 

This  district  is  in  the  heart  of  one  of  the  highest  and  most  rugged 
mountain  regions  of  Idaho.  It  is  accessible  only  by  trail,  though  n 
good  mountain  road  is  possible  via  Beaver  and  Bernard  creeks  from  a 
point  on  the  State  wagon-road  in  the  vicinity  of  Gape  Horn.  The  center 
of  the  district  is  a,  hill  7  or  8  miles  down  Bernard  Creek,  north  of  the 
divide  between  the  upper  Salmon  and  the  Middle  Fork.  The  periphery 
of  the  district,  however,  includes  the  divide,  aud  along  this  there  has 
been  considerable  prospecting;  owing  to  misinformation,  it  was  the 
only  locality  visited,  and  the  following  description  refers  entirely  to  it. 

The  country  rock  is  gray  granite  and  quartzitic  and  micaceous 
schists,  which  in  some  layers  are  strongly  calcareous.  The  schists 
occur  both  in  series  by  themselves  and  as  included  belts,  1  to  100  feet 
or  more  wide,  in  the  granite.  The  latter  is  more  commonly  the  occur- 
rence on  the  summit  of  the  divide,  while  the  schists  in  series  are  found 
chiefly  on  the  northern  slope.  The  bauds  of  especial  interest  here  are 
those  in  the  granite.  They  may  be  traced  for  a  mile  or  more,  or  may 
disappear  after  a  few  hundred  feet,  seeming  to  occur  as  irregular,  iso- 
lated bodies.  This  appearance  may  be  due  to  original  differentiation 
in  the  material  from  which  the  granites  and  schists  were  derived  or  to 
included  masses  through  faulting,  the  former  perhaps  being  most 
plausible.  The  strike  of  schists,  and  of  the  granites  where  they  show 
foliation  planes,  varies  to  all  points  of  the  compass,  N.  15°  to  35°  W.,  N. 
CO0  E.,  and  east  and  west,  being  most  common.  This  variation  is  due 
to  local  flexures,  evidences  of  which  are  constantly  recurring.  The  dip  of 
the  series  in  the  divide  is  to  the  west  or  south,  usually  at  a  high  angle. 

Diorite-porphyrites  and  quartz-porphyrites  occur  in  dikes  through- 
out a  large  territory  about.  In  the  immediate  region  examined  the 
quartz-porphyrites  are  most  common;  their  trend  is  often  with  that  of 
the  slates,  though  occasionally  across  it.  They  occur,  in  some  instances, 
in  proximity  to.  if  not  immediately  next,  the  ore  bearing  slates;  in 
others  they  are  at  a  distance.  Their  influence  upon  the  ore  bodies 
could  not  be  ascertained  from  the  few  shallow  prospects  existing. 

The  ore  bodies  are  the  result  of  the.  mineralization  of  certain  layers 
in  the  zones  of  slates  or  schists.  Quartzitic,  micaceous,  and  calcareous 
beds  all  carry  ore  in  greater  or  less  amount.  If  the  zone  of  slates  is 
narrow— rl  to  2  feet — the  entire  width  may  be  more  or  less  mineralized; 
in  this  case  the  walls  are  granite,  in  the  wider  belts  the  ore  shows  in 
considerable  deposits  in  some  of  the  layers,  in  others  in  much  less 
amount;  in  others  still  it  may  be  altogether  wanting. 

Evidences  of  fissures  were  slight,  though  in  some  instances  the 
rocks  were  contorted  to  a  considerable  degree. 

The  ores  are  argentiferous  galena,  and  autimonial  and  arsenical  sul- 
phides, also  silver-bearing,  probably.  They  are  more  or  less  altered 
near  the  surface.  Their  value  varies  greatly,  but  in  the  assorted  ore 
it  is  sufficient  to  warrant  shipping  by  pack  train. 


ELDBIDQE.J  YELLOW   JACKET   DISTRICT.  259 

THE    YELLOW   JACKET   DISTRICT. 

This  district  lies  southwest  of  Salmon  City,  on  Yellow  Jacket  Creek, 
a  tributary  of  Cainas,  which  in  turn  enters  the  Middle  Fork  of  Salmon 
Kiver.  It  is  in  the  midst  of  mountains  of  great  ruggedness  and  is 
approached  by  trail  from  Challis  and  Salmon  City,  each  about  GO  miles 
distant.  From  the  latter  place,  however,  a  good  road  has  just  been 
completed  via  Leesburg  to  the  mouth  of  Fourth  of  July  Creek,  12 
miles  from  the  mining  camp.  The  outward  communication  from  Sal- 
mon City  is  by  wagon  road  75  miles  to  lied  Eock,  on  the  Utah  North- 
ern Railroad;  from  Challis  by  wagon  road  75  miles  to  Ketehum,  the 
terminus  of  a  branch  of  the  Oregon  Short  Line.  The  altitudes  above 
sea-level  along  the  Salmon  City  road  west  are  given  below. 

Altitudes  between  Salmon  City  and  the  Yellow  Jacket  district,  Idaho. 

Feet. 

Salmon  City,  about 4,000 

Divide  between  the  waters  of  the  main  Salmon  and  Big  Creek 8,500 

Leesbnrg 6,550 

Big  Creek  at  the  mouth  of  Napias 5,500 

Mouth  of  Fourth  of  July  Canyon 5,900 

Divide  between  the  waters  of  Big  Creek  and  Yellow  Jacket  Creek 7,800 

Yellow  Jacket  Camp 5,875 

The  readings,  which  are  from  an  aneroid,  if  out,  are  a  little  too  high. 

The  Yellow  Jacket  camp  is  10  to  15  miles  below  the  head  of  the  basin 
drained  by  Yellow  Jacket  Creek  and  its  upper  tributaries.  The  gorge 
of  the  main  creek  at  the  camp  is  about  one-fourth  of  a  mile  wide, 
closing  to  50  or  75  yards  just  above,  with  occasional  narrow  openings 
still  higher.  Below  the  camp  the  width  of  one-fourth  to  one-half  of  a 
mile  is  maintained  for  at  least  a  mile  or  two,  and  along  this  portion  are 
gold  placers  in  preparation  for  being  worked.  The  sides  of  the  gorge 
are  comparatively  steep,  and  the  country  about  is  well  timbered.  Two 
or  three  gulches  enter  the  Yellow  Jacket  at  the  camp,  one  from  the 
south,  the  others  from  the  north,  the  latter  short,  though  reaching  well 
toward  the  summit  of  the  ridge.  The  two  great  lode  properties  are 
those  of  the  Columbia  Consolidated  Gold  Mining  Company  and  the 
Yellow  Jacket  Mining  Company.  Other  claims  are  located  but  unde- 
veloped. The  Columbia  property  is  northwest  of  the  main  stream,  in 
the  divide  between  this  and  its  tributary,  Hoodoo  Creek.  The  east 
side  of  this  divide  is  that  on  which  present  exploitation  is  conducted. 
It  presents  a  steep,  slightly  indented  slope,  advantageous  for  the  open- 
ing of  mines.  The  Yellow  Jacket  lode  is  between  1  and  2  miles  north- 
east of  the  Columbia  and  also  north  of  the  main  stream.  It  is  located 
well  up  the  slope  of  a  steep  hill,  the  southern  terminal  of  a  spur  from 
a  massive  ridge  in  the  curve  of  Yellow  Jacket  Creek.  This  property 
is  separated  from  the  Columbia  by  the  two  lateral  gulches  mentioned 
above.  The  mining  region  in  its  entirety  extends  to  the  east  for  a  mile 
or  two  up  Yellow  Jacket  Creek  in  the  hills  on  either  side,  and  to  the 


260 


GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 


west  about  the  heads  of  Hoodoo,  Lake,  and  Wilson  creeks.  West  of 
Wilson  Creek  is  the  divide  separating  it  from  the  Middle  Fork  of  the 
Salmon.  South  and  southeast  of  the  Yellow  Jacket  camp  the  ridge 
separating  it  from  Camas  Creek  and  its  tributary,  Silver  Creek,  is  high 
and  rugged,  its  topographic  configuration  being  most  irregular.  The 
ridge  is  the  site  of  great  volcanic  activity  in  past  time. 

Yellow  Jacket  Creek  is  a  small  stream,  but  the  water  is  sufficient  for 
milling  and  other  purposes  in  connection  with  vein  mining.  For  the 
placers  it  must  be  utilized  chiefly  in  the  early  part  of  the  season. 


FIG.  40.— Sketch-map  of  Yellow  Jacket  district,  by  F.  D.  Howe. 


The  geology  of  the  Yellow  Jacket  region  was  but  generally  examined 
in  the  reconnaissance.  The  district  lies  well  within  the  area  of  crys- 
talline schists,  but  local  exposures  of  the  gray  granite  so  common  in 
Idaho  occur  in  the  ranges  about,  some  of  them  probably  of  consider- 
able area.  It  is  reported,  for  instance,  in  extended  outcrop  on  the  head 
of  the  main  fork  of  Yellow  Jacket  Creek,  10  to  15  miles  above  the  camp, 
in  the  divide  between  the  waters  of  the  Salmon  and  the  Middle  Fork. 
Abundant  debris  from  this  source  shows  along  the  creek  bottom.  It 
is  also  said  to  occur  a  few  miles  west  of  the  district,  along  and  beyond 


ELDKIDOE.]  YELLOW   JACKET   DISTRICT.  261 

the  Middle  Pork,  a  statement  borne  out  by  the  appearance  of  the  topog- 
raphy in  that  direction.  The  prevailing  rocks,  however,  are  the  crys- 
talline schists,  quartzitic  and  micaceous  chiefly,  but  with  occasional 
bauds  more  or  less  calcareous.  The  quartzitic  variety  predominates, 
and  in  one  instance,  in  a  layer  of  typical  quartzite,  distinct  ripple-marks 
were  observed  on  the  stratification  planes.  The  series  is  thin  bedded 
throughout. 

The  eruptive  rocks  include  rhyolite,  trachyte,  andesite,  quartz- 
porphyry,  mica-diorite,  syenite,  aphauitic  syenite  and  iniuette  (lain- 
prophyre),  and  diabase.  Of  these  the  quartz-porphyries,  rhyolites,  and 
andesites  occur  in  the  heaviest  and  apparently  most  irregular  bodies ; 
the  others,  usually  as  narrower  dikes.  The  quartz-porphyry,  and  the 
aphanitic  syenite  and  minette  (lamprophyres),  called  "  syenite"  by  the 
miners,  are  most  commonly  associated  with  the  metalliferous  veins. 
The  lamprophyres,  according  to  the  observations  of  Mr.  F.  D.  Howe, 
manager  of  the  Columbia  Consolidated  Gold  Mining  Company,  when 
present  and  in  contact  with  the  veins,  are  usually  their  foot  wall,  while 
the  hanging  wall  may  consist  of  quartzite,  quartz- porphyry,  or  some 
other  one  of  the  eruptives.  The  lainprophyre  dikes  are  considered  as 
indicative  of  the  near  presence  of  an  ore  body,  so  persistent  is  their 
occurrence  in  this  connection ;  at  least  this  is  the  case  on  the  Columbia 
Hill.  On  the  Yellow  Jacket  Hill  the  lamprophyres  do  not  seem  to  occur 
so  often  nor  in  such  intimate  connection  with  the  veins;  indeed,  the 
veins  themselves  differ  in  the  two  hills.  The  diabase  was  observed 
only  on  the  Yellow  Jacket  Hill.  The  andesite  occurred  on  the  divide 
between  the  Fourth  of  July  and  Yellow  Jacket  drainage,  while  the 
rhyolites  were  most  conspicuous  1  to  2  miles  east  of  the  camp  and  on 
Fourth  of  July  Creek.  Quartz-porphyries  in  heavy  masses  occurred 
both  at  a  distance  and  on  the  crest  of  the  ridge  just  above  the  Colum- 
bia miues.  Occasionally  a  dike  appears  to  be  interbanded  with  the 
schists,  but  in  most  cases  they  cross  the  dips.  The  influence  of  the 
eruptives  upon  the  mineralization  of  the  region  is  undetermined. 

The  entire  region  between  Panther  and  Camas  creeks,  which  includes 
the  Yellow  Jacket  mining  district,  has  been  in  former  times  one  of  great 
dynamic  disturbances;  folds,  flexures,  and  faults,  in  addition  to  the 
intrusion  of  the  eruptives,  occur.  The  structure  is  therefore  difficult 
to  determine,  and  the  difficulty  is  increased  by  the  similarity  of  the 
schists  from  the  base  to  the  summit,  rendering  identification  of  hori- 
zons impossible.  In  general  the  structure  as  suggested  by  strikes 
seems  to  have  been  developed  on  lines  varying  from  N.  30°  E.  to  N.  20° 
W.,  with  local  divergences  to  N.  60°  E.  and  N.  60°  W.  This  is  evident 
both  in  the  divide  between  Panther  and  Yellow  Jacket  creeks  and  the 
country  to  the  west,  to  and  beyond  the  Yellow  Jacket  mining  camp. 
Moreover,  it  is  on  these  lines  that  the  majority  of  the  eruptives  have 
cut  through,  although  there  are  doubtless  instances  in  which  the  dikes 
cross  the  schists  at  greater  or  less  angles  with  their  strike.  Again,  in 


262      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

some  of  the  folds,  on  both  small  ami  large  scale,  slates  and  dikes  together 
appear  to  have  swerved  from  the  normal  or  more  usual  trend. 

The  dips  in  the  Panther- Yellow  Jacket  di  vide,  along  the  route  traveled, 
are  nearly  all  steep.  On  the  eastern  side  the  beds  more  commonly 
incline  to  the  east  when  departing  from  the  vertical,  and  this  direction 
becomes  pronounced  along  Big  Creek  between  Fourth  of  July  and 
Napias,  where  dips  as  low  as  20°  are  sometimes  encountered,  from  20° 
to  35°  being  quite  common.  On  the  western  side  of  the  divide  a  verti- 
cal position  is  maintained  by  much  of  the  series,  but  in  instances  of 
departure  from  this  a  westerly  dip  is  perhaps  most  usual,  though  a 
steep  easterly  inclination  is  occasionally  met  with.  The  westerly  dip 
becomes  more  pronounced  on  approaching  the  vicinity  of  the  Yellow 
Jacket  camp,  notwithstanding  that  In  this  immediate  region  the  strata 
have  been  extensively  fractured  and  crumpled,  with  consequent  varia- 
tion in  both  strike  and  dip.  West  of  the  Yellow  Jacket  camp  the 
structure  lines  of  the  formations  are  unknown. 

In  general  it  appears  from  the  foregoing  observations  that  the  high 
range  forming  the  Panther- Yellow  Jacket  divide  marks  the  axis  of  an 
anticline,  and  that  on  the  west  side,  especially,  the  rocks  have  been 
locally  fractured  and  bent,  affording  an  opportunity  for  the  locus  of 
mineralization  in  the  hills  about  the  Yellow  Jacket  camp. 

In  the  Columbia  Hill  the  schists  are  particularly  crumpled  and  frac- 
tured. There  are  several  lines  of  structure,  all  bearing  upon  the 
geology  of  the  ore  bodies.  Three,  N.  20°  W.,  N.  15°  E.,  and  K  30°  E., 
are  strikes  of  the  schists,  representing  flexures  in  their  trend.  Occa- 
sionally these  are  also  the  courses  of  joint  planes.  Two  other  lines  of 
structure,  N.  GO0  E.  and  N.  15°  to  25°  W.,  prevail  throughout  the 
region,  the  directions  of  joint  planes  and  perhaps  also  of  strikes  of  the 
schists.  These  directions  for  the  joint  planes  become  especially  sig- 
nificant when  it  is  known  that  they  are  also  the  directions  of  the  two 
chief  systems  of  ore  deposits  of  this  hill,  the  veins  of  a  N.  60°  E. 
trend  forming  the  principal  system.  The  observed  dips  of  the  locality 
were  all  to  the  west,  varying  from  10°  to  60°  and  even  90°. 

In  the  Yellow  Jacket  Hill,  a  mile  or  two  northeast  of  the  Colum- 
bia, the  strike  of  the  schists  is  N.  50°  to  CO0  E.,  and  this  is  here  the 
strike  of  the  main  vein  of  the  property  also.  Schists  and  vein  together 
have  an  average  dip  of  33°  W.,  locally  increasing  to  45°,  50°,  or  even 
70°  or  90°,  but  the  steepest  dips  are  rare.  Faults  doubtless  occur  in 
both  the  Columbia  and  Yellow  Jacket  hills,  but  their  relations  to  the 
ore  bodies,  eruptives,  and  country  rock,  in  strike  and  dip,  were  not 
determined. 

The  principal  system  of  metalliferous  veins  in  the  Columbia  Hill — 
that  having  the  N.  60°  E.  trend — appears  to  be  a  series  of  mineralized 
zones  of  highly  fractured  material,  in  width  from  a  few  feet  up  to 
possibly  75  or  100;  50  was  the  maximum  observed  by  the  writer.  Their 
dip  varies  between  30°  and  70°  W.  The  foot  walls  are  frequently 


ELDEIDOE.]  YELLOW   JACKET   DISTRICT.  263 

aphauitic  syeniteor  ruinette  (lamprophyre;  local  term,  "syenite"),  while 
the  baiigirig  wall  may  be  either  quartzite,  quartz-porphyry,  or  some 
other  eruptive,  occasionally  even  the  syenitic  rock.  The  eruptives  in 
general  parallel  the  veins  in  strike,  and,  in  some  instances  at  least,  the 
qnartzites  also  seem  to  do  so. 

Tiie  fractured  zones  are  clearly  defined,  but  it  was  not  possible  in 
the  time  available  for  the  examination  to  determine  satisfactorily 
whether  they  were  at  all  points  parallel  with  the  line  of  strike  of  the 
schists  or  in  part  independent  of  and  divergent  from  this.  Again,  it 
was  equally  impossible  to  determine  in  the  yet  comparatively  shallow 
openings  whether  the  zones,  when  coincident  with  the  schists  in  strike, 
dipped  with  them  or  at  a  greater  or  less  angle.  Whichever  may  be 
the  case,  it  will  not  alter  the  character  of  the  deposit.  The  vein 
material  is  a  breccia  of  fractured  schist,  in  one  or  two  instances  includ- 
ing pieces  of  the  accompanying  lamprophyre  ("syenite").  The  breccia 
is  bound  together  with  siliceous  matter  (quartz),  and  the  filling  com- 
pleted with  the  mineralization  of  the  zone.  From  the  included  lam- 
prophyre it  would  seem  that  at  least  the  fissures  it  filled  existed  prior 
to  the  fracturing  which  resulted  in  the  brecciated,  mineralized  zones. 
In  regard  to  the  other  eruptives,  the  time  relations  are  obscure. 

The  metallic  contents  of  the  fractured  zones  are  carried  not  only  in 
the  interstitial  quartz  but  often  in  the  fragments  of  the  schists  and 
slates  themselves,  impregnation  of  these  having  taken  place  to  consid- 
erable depths,  though  perhaps  not  to  the  complete  replacement  of  the 
central  portion.  The  exterior  of  such  fragments  under  atmospheric  or 
other  oxidizing  agencies  often  appears  rusty,  from  the  partial  alteration 
of  the  sulphides  to  oxides  or  carbonates.  The  deposition  of  the  metallic 
contents  has  been  more  or  less  unequal  throughout  the  lodes. 

The  system  of  veins  on  the  Columbia  property  having  a  N.  15°  to  25° 
W.  trend  is  less  prospected  than  that  just  described.  Such  veins  have 
in  a  few  instances  been  observed  coming  into  one  or  another  of  those  of 
the  N.  60°  E.  system,  but  their  extent  in  the  line  of  their  trend  is  unde- 
termined. Neither  could  it  be  observed  at  the  time  of  the  examination 
.whether  at  all  points  these  were  confined  to  the  areas  in  which  a  N.  15° 
to  25°  W.  strike  prevailed  for  the  schists;  whether,  in  fact,  they  did  not, 
as  suggested  in  one  or  two  instances,  form  lateral  "feeders"  to  or  ott- 
shoots  from  the  X.  60°  E.  system,  on  the  stratification  or  other  planes, 
or  even  as  fissures.  It  is  significant  that  joint  planes  having  a  direc- 
tion of  N.  25°  W.  are  of  frequent  occurrence  throughout  the  property 
of  the  Columbia  Company. 

The  ores  of  the  Columbia  mines  are  chiefly  copper  sulphides  carrying 
gold  and  silver  and  ranging  in  value  up  to  $150  per  ton,  with  concen- 
trates increasing  in  richness  according  to  the  completeness  of  the 
operation.  An  average  ore  is  stated  to  be  about  $30,  while  $40  to  $50 
is  not  infrequent.  The  sulphides  near  the  surface  are  more  or  less 
oxidized,  or  altered  to  carbonates,  affording  a  free  milling  ore ;  and  it 


264  GEOLOGICAL   RECONNAISSANCE   ACROSS   IDAHO. 

is  stated  that  there  is  more  or  less  free  gold  iii  the  unaltered  portions 
also. 

In  the  hill  embracing  the  property  of  the  Yellow  Jacket  Gold  Mining 
Company  there  appears  to  be,  so  far  as  shown  by  the  present  state  of 
exploitation,  but  a  single  vein,  or  at  most  two,  if  we  except  a  number 
of  small  stringers  in  their  vicinity.  The  country  is  a  quartzitic  schist 
in  layers  of  varying  thickness  and  hardness.  The  veins  are  of  quartz, 
and  lie  with  the  schists,  striking  N.  60°  E.,  and  dipping  northwest  on 
an  average  33°,  increasing  at  points  to  45°,  60°,  or  even  00°.  But  a 
single  instance  of  a  breccia  was  observed  in  the  superficial  examination 
given  the  mine;  this  consisted  of  included  fragments  of  country  rock 
in  a  calcite  cement.  The  highly  brecciated  condition  prevailing  in  the 
Columbia  mines  appears  to  be  absent  in  the  Yellow  Jacket.  The  vein 
has  been  traced  but  a  short  distance  beyond  the  Yellow  Jacket  Hill. 
The  thickness  of  the  vein  is  said  to  reach  40  feet  locally,  but  the  maxi- 
mum seen  by  the  writer  was  about  15  feet.  The  quartz  is  said  to 
occur  in  lenticular  bodies  of  varying  dimensions.  Occasionally  erup- 
tive dikes  appear  in  the  mine,  chiefly  of  aphanitic  syenite  or  "minette" 
(lamprophyre)  and  diabase.  Their  relations  to  the  ore  bodies  were  not 
studied. 

The  ore  of  the  Yellow  Jacket  mine,  or  at  least  of  that  portion  of  it 
visited  by  the  writer,  is  a  free-milling,  auriferous  quartz,  with  one  or 
two  small  local  bodies  of  hematite  and  an  occasional  but  rare  copper 
stain.  This  difference  from  the  ore  of  the  Columbia  mine  is  as  marked 
as  the  difference  in  the  manner  of  occurrence  and  character  of  the 
veins  themselves.  The  mass  of  the  ore  of  the  Yellow  Jacket  mine  is 
said  to  run  from  $7  to  $30  per  ton,  with  local  values  much  higher.  An 
ore  of  common  occurrence  is  one  of  $18  or  $19. 

The  formation  about  the  summit  of  the  Yellow  Jacket  Hill  has 
undergone  considerable  disintegration,  and  the  surface  is  covered  with 
debris  which  is  reported  as  carrying  free  gold  from  a  trace  up  to  $7 
per  ton.  This  may  be  milled  at  a  profit. 

THE   WOOD   RIVER  DISTRICT. 

This  district  embraces  an  area  15  to  25  miles  square  on  upper  Wood 
Eiver  and  adjoining  streams,  just  within  the  southern  edge  of  the 
mountain  mass  of  Idaho.  The  greater  portion  of  the  district  lies 
within  the  confines  of  the  Wood  River  drainage.  This  has  formed  a 
topographic  depression  ridged  with  spurs  of  the  inclosing  ranges,  the 
intervening  gulches  being  sharply  eroded  to  depths  of  1,500  to  3,000 
feet.  The  central  valley,  one-fourth  to  one-half  mile  wide,  ranges  in 
altitude  from  5,000  feet  at  its  lower  end  (Bellevue)  to  6,000  feet  a  little 
above  Ketchum,  rising  still  more  rapidly  beyond.  The  periphery  has 
an  elevation  of  9,000  to  10,000  feet,  a  single  peak,  Mount  Hyndman, 
15  to  20  miles  southeast  of  Ketchum,  reaching  12,000  feet,  the  highest 
in  the  State. 


U.   S.   GEOLOGICAL  SURVEY 


SKETCH   MAP  OF  WOOD   RIVER   DISTRICT. 


ELDEIDQE.]  WOOD    RIVER   DISTRICT.  265 

The  ranges  and  their  spurs  are  somewhat  rugged,  and  their  configu- 
ration varies  according  as  the  rocks  entering  into  their  composition  are 
quartzites,  limestones,  or  shales.  Most  of  the  streams  are  comparatively 
small,  but  Wood  River  has  a  large  volume  of  water.  The  bottom  lands 
along  the  main  stream  are  cultivated  and  supply  the  mining  camps ; 
the  hills  are  timbered  in  varying  amount,  though  the  mines  are  not 
always  in  close  proximity  to  the  heavier  bodies.  Ilailey  and  Ketchum, 
12  miles  apart,  are  the  important  towns  of  the  district,  and  are  connected 
by  a  branch  with  the  Oregon  Short  Line  at  Shoshone. 

A  resume  of  the  general  geology  for  the  district  shows  a  base  of  gray 
granite;  resting  upon  this,  in  one  locality,  dark  and  light  colored  quartz- 
ites with  calcareous  beds;  in  another,  a  heavy  mass  of  pink  quartzite, 
varying  in  thickness  up  to  500  feet;  and  in  a  third,  a  great  series  of 
black  or  dark-gray  calcareous  shales.  Intercalated  somewhere  in  this 
series,  probably  between  the  gray  shales  last  mentioned  and  the  dark 
quartzitic  series,  are  from  300  to  500  feet  of  heavy- bedded  limestone, 
resembling  somewhat  the  sub-Carboniferous  of  the  Eocky  Mountains. 
The  arrangement  suggested  as  most  probable,  in  the  light  of  the  evi- 
dence at  hand,  is  granite,  heavy  pink  quartzite  (Cambrian),  dark  and 
light  quartzitic  series  (at  least  post-Cambrian),  the  limestone  resembling 
the  sub-Carboniferous,  and  the  gray  calcareous  shales  (Carboniferous). 
All  these  are  cut  by  eruptives — horublende-andesite  and  hombleude- 
mica-audesite.  The  upper,  calcareous,  shaly  division  carries  most  of 
the  mines  of  the  Wood  Eiver  district  proper,  but  in  the  outlying 
portions  the  dark  quartzite  series  and  the  granite  also  contain  mineral 
deposits  of  one  kind  or  another. 

Throughout  the  Wood  Eiver  region  proper  there  is  a  general  N.  25° 
to  40°  W.  strike  with  a  southwest  dip,  usually  between  30°  and  00°; 
but  local  variations  occur,  sometimes  involving  a  region  of  considerable 
importance.  The  southwest  dip  is  that  of  the  half  of  a  probable  anti- 
cline, the  eastern  half  being  in  the  Lost  Eiver  country,  and  the  axis 
along  the  divide  between  this  and  Wood  Eiver. 

Faults  are  of  frequent  occurrence  throughout  the  region.  They 
are  of  many  directions  and  of  varied  inclination  and  throw.  The 
amount  of  throw  is  often  obscured  by  the  similarity  of  the  strata  on 
either  side  of  the  fracture  plane  and  by  the  great  thickness  of  the 
series  opposed.  Following  are  the  types  observed,  which  are  probably 
representative  for  the'  region. 

One,  the  plane  of  which  is  horizontal,  displacing  a  dipping  ore  body 
50  feet  or  more;  the  upper  part  of  the  vein  is  carried  in  the  direction 
of  the  dip,  both  ends  of  it  being  broken  over,  with  a  dragging  of  the 
ore  along  the  plane  of  the  fault.  Faults  inclined  25°  to  50°  also  occur, 
their  strikes  often  divergent;  such  faults  are  known  to  have  drained 
the  upper  levels  of  a  mine  when  struck  at  a  lower.  These  faults  may 
sometimes  be  traced  at  the  surface  by  a  break  in  the  topography,  a  line 
of  gentle  depression  or  otherwise.  It  is  often  significantly  remarked 


266      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

by  the  miners  of  various  districts  that  the  ore  follows  the  line  of  a 
gulch.  The  position  of  a  gulch  is  often  determined  by  a  fracture-line, 
and  it  is  along  a  fracture-plane  that  mineral  solutions  have  found  their 
way;  hence  the  coincidence.  Fracturing  has  also  taken  place  with  a 
minimum  of  displacement,  at  times  no  more  than  a  crushing  of  a  par- 
ticular zone  with  a  slight  slipping  of  fragments  upon  one  another  or  upon 
the  walls.  This  was  observed  both  coincident  with  the  stratification 
and  across  it.  Slickensides  are  observed  nearly  everywhere  that  there 
has  been  movement  between  the  rocks  on  the  two  sides  of  the  faults, 
the  grooves  lying  in  many  directions.  In  addition  to  the  above  recog- 
nized faults  there  are  many  points  along  Wood  River  and  its  tributaries 
where  the  series  of  calcareous  shales  opposes  the  underlying  heavy- 
bedded  gray  limestone,  which  is  indicative  of  considerable  throws.  The 
faults  of  the  Wood  Eiver  region  are  among  the  most  important  fea- 
tures requiring  examination.  Flexures  also  occur,  but  study  of  their 
details  was  not  attempted. 

Mineralization  of  the  veins  in  the  Wood  liiver  district  has  appar- 
ently taken  place  in  a  manner  wholly  independent  of  the  eruptives. 
The  region  has  at  different  periods  been  one  of  great  fracturing,  on 
both  a  large  and  a  small  scale,  and  passages  have  thus  been  afforded, 
in  some  instances  for  the  intrusion  of  dikes  and  other  irregular  bodies 
of  eruptives,  in  others  for  the  flow  of  mineral -bear  ing  solutions.  These 
solutions  have  not  only  filled  the  fractured  zones  themselves  but  have 
oftentimes  found  their  way  into  the  more  solid  beds  of  limestone  adjoin- 
ing, replacing  them  to  such  a  degree  that  they  too  became  of  economic 
value. 

The  mines  of  the  Wood  Kiver  region  that  have  thus  far  been  most 
productive  lie  within  the  periphery  of  the  drainage  basin  itself.  A 
single  exception,  perhaps,  is  the  Camas  No.  2,  near  Doniphan,  just 
beyond  the  southwestern  border.  The  position  of  the  mines  may  be 
isolated,  or  there  may  be  certain  localities  where  the  deposition  of  ores 
has  been  especially  active,  possibly  through  an  easier  access  afforded 
the  mineral-bearing  solutions.  For  lack  of  time  it  was  impossible  to 
do  more  than  (under  the  guidance  of  two  or  three  of  the  mining  men 
acquainted  with  the  district)  to  visit  a  few  localities  which  together 
would  afford  a  general  idea  of  the  occurrence  of  the  ores  and  of  their 
character. 

On  both  sides  of  Wood  Kiver,  fracturing,  faulting,  and  folding  were 
suggested  in  the  distribution  of  the  strata  and  their  many  variations 
in  strike  and  dip.  For  the  region  east  of  the  main  valley  the  general 
anticlinal  structure  described  for  the  southeastern  portion  of  the  Saw- 
tooth Range  prevails.  The  strike  most  common  is  N.  25°  to  40°  W., 
the  dip  usually  southwest.  Faults  of  greater  or  less  extent  and  mag- 
nitude may  be  seen  both  in  the  mines  and  upon  the  surface,  and  folds 
were  discerned  from  a  distance  by  the  writer.  The  area  of  examination, 
however,  was  very  small.  Om  the  west  side,  the  region  about  the  head 


WOOD    RIVER   DISTRICT.  267 

of  Deer  Creek  and  its  tributaries,  extending  over  to  Bullion  Creek  and 
the  gulches  west,  received  most  attention.  It  is  9  to  10  miles  west  of 
Hailey,  and  2,000  to  2,500  feet  higher.  The  geology  along  lower  Deer 
Creek  is  obscure  and  subject  to  sharp  changes  in  both  structure 
and  stratigraphy.  South  of  the  creek  limestones  bearing  a  close 
resemblance  to  the  heavy-bedded  gray  variety  of  the  district  appear 
in  the  bluffs  in  one  or  more  flexures,  while  eruptives  form  a  portion  at 
least  of  the  higher  hills  beyond.  On  the  north,  for  the  first  mile,  an 
eruptive  closely  resembling  the  horublende-andesite  higher  up  Wood 
Eiver  occupies  the  interval  opposite  the  flexed  limestones  ou  the  south. 
The  eruptive  finally  gives  way  to  the  dark  quartzitic  series ;  this  in  a 
mile  or  two  to  a  heavy-bedded  blue  limestone,  which  also  now  appears 
on  the  south  side  of  the  valley.  In  about  200  yards  the  limestone  is 
succeeded  by  the  black  and  gray  calcareous  shales,  here  more  calcare- 
ous than  at  many  points  in  the  district.  The  general  strike  of  this 
series  is  across  the  creek,  the  dip  probably  upstream,  or  west. 

At  the  Warm  Springs,  which  are  about  4  miles  above  the  mouth  of 
Deer  Creek,  the  slates  are  succeeded  by  gray  granite;  it  is  impos- 
sible to  state  whether  in  natural  sequence  or  by  faulting,  but  the  latter 
is  suspected.  The  granite  extends  for  3J  miles  along  the  creek,  the 
foliation  planes  having  the  general  strike  of  the  country,  N.  25°  W.,  and 
dipping  upstream,  westward.  The  granite  is  overlain  by  500  feet  of 
white  quartzite,  which,  however,  disappears  within  a  mile  or  two  south 
of  the  creek,  the  manner  of  disappearance  not  being  determined.  This 
is  overlain  by  the  black  and  gray  calcareous  shales  and  limestones,  the 
ore- bearing  series  of  the  Wood  Eiver  region,  which,  where  the  quartzite 
has  disappeared,  at  the  head  of  Narrow  Gauge  and  Bullion  gulches, 
come  directly  down  on  the  granite.  The  dip  of  the  entire  section, 
from  granite  up,  is  westward  until  at  the  head  of  Deer  Creek  and  of  its 
tributary  gulch,  Eed  Cloud,  it  gradually  changes  to  the  southwest  and 
south,  the  strike  of  the  slates  changing  accordingly.  This  departure 
in  the  strike  and  dip  is  particularly  conspicuous  in  the  high  ridge  sepa- 
rating Eed  Cloud  Gulch  from  the  gulches  to  the  south  in  which  the  Eed 
Elephant  and  Bullion  group  of  mines  are  located.  For  all  this,  how- 
ever, it  is  a  local  feature,  and  the  normal  strike  of  N.  25°  W.  prevails 
over  much  of  the  area  included  within  this  southwestern  corner  of  the 
Wood  Eiver  district. 

The  hill  in  which  the  Red  Cloud,  Eed  Elephant,  Bullion,  and  French 
groups  of  mines  are  opened  is  one  of  marked  fracturing  as  well  as 
folding,  both  on  a  large  and  small  scale.  The  fractures  occur  at  angles 
divergent  Avith  the  strikes  and  dips,  and  also  along  their  planes,  and 
are  locally  so  complex  as  completely  to  obscure  the  bedding.  No  clearly 
defined  system  of  fracturing  has  thus  far  been  worked  out  for  the  hill, 
but  that  most  prominent  has  a  N.  25°  W.  direction.  This  also  carries 
several  of  the  more  important  mines.  The  greatest  irregularity  appears 
to  be  in  the  northern  face  of  the  hill.  lu  the  southern,  or  at  least  iu 


268      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

the  spurs  on  this  side,  the  strata  seems  to  have  regained  the  strike 
normal  for  the  district  in  general,  N.  25°  W.,  the  dip  being  uniformly 
westward,  30°  to  60°,  or  locally  even  nearer  the  vertical.  The  precise 
connection  between  the  structure  of  the  northern  and  southern  faces  of 
the  hill  was  not  determined. 

The  hill  just  described  is  on  the  very  edge  of  the  great  mountain  mass 
of  Idaho,  but  few  foothills  intervening  between  it  and  the  lava  plains 
of  the  Camas  and  Snake  valleys.  Neither  the  region  at  the  very  head  of 
Deer  Creek  and  its  tributary,  Red  Cloud  Gulch,  nor  tue  country  to  the 
west  of  this  was  visited.  It  is  said  that  granite  lies  beyond  the  Wood 
Eiver  drainage,  in  this  direction,  and  the  observations  of  the  writer 
from  a  distant  point  to  the  south  confirm  this  in  part.  It  is  quite  possi- 
ble that  such  succession  of  granites  west  of  the  Deer  Creek  shales  and 
slates  is  brought  about  by  a  bending  up  of  the  strata  to  the  west, 
making  a  syncline  somewhere  between  the  Deer  Creek  mines  and  the 
granite.  Faulting  is  the  alternative. 

The  ore  deposits  of  the  Wood  Eiver  region,  so  far  as  observed  by 
the  writer,  are  of  three  classes :  those  occupying  fractured  zones  in  the 
great  body  of  calcareous  shales  and  limestone;  those  occurring  in  beds 
of  limestone ;  and  typical  fissure  veins.  There  are  instances,  however, 
in  which  these  classes  seem  to  occur  in  connection  one  with  another. 

The  fractured  zones  are  more  or  less  prominent  throughout  the  dis- 
trict. Such  zones  have  the  appearance  of  a  breccia  of  the  country 
rock,  with  the  interstices  filled  with  quartz,  not  infrequently  supple- 
mented by  calcite  in  minor  amounts,  the  whole  more  or  less  impreg- 
nated with  the  metallic  contents.  The  width  of  such  zones  varies  from 
a  foot  or  two  up  to  15  or  20  feet,  and  may  be  even  greater  locally. 
The  zones  may  coincide  with  a  particular  bed  of  limestone  or  may  cut 
across  both  strike  and  dip  at  various  angles  of  divergence.  When 
this  divergence  is  slight  it  is  difficult  to  distinguish  between  an  ore- 
bearing  bed  and  an  independent  zone  of  fracture,  except  through  the 
breccia  structure.  What  are  called  foot  and  hanging  walls  are  also 
recognized  in  veins  of  this  kind.  Where  the  fractured  zone  crosses  the 
strata,  occasionally  a  lateral  ore-shoot  follows  a  bedding  plane  for  a 
short  distance,  given  off  from  the  main  channel  of  mineralization; 
indeed,  mineralization  along  bedding  planes,  however  slight,  is  con- 
sidered an  indication  of  the  near  approach  to  a  vein. 

The  fractured  zones  may  have  resulted  from  simple  crushing  with  a 
minimum  of  movement;  or,  as  is  apparently  often  the  case,  the  move- 
ment may  have  been  considerable.  The  creation  of  the  spaces  now 
occupied  by  interstitial  filling  may  have  been  by  actual  separation  of 
the  fragments;  or  they  may  have  been  merely  a  network  of  joints  and 
cross-joints  along  which  solvents  easily  passed,  carrying  away  portions 
of  the  lime  and  so  creating  interstices  which  were  subsequently  filled 
with  the  mineralizing  solutions;  or,  perhaps,  even  the  calcareous 
material  of  the  breccia  itself  was  gradually  replaced  by  the  metallic 


KLDKIUOE.]  WOOD    RIVER    DISTRICT.  269 

compounds.  When  considerable  movement  took  place  in  the  zone  of 
fracture,  fissures  were  often  created  within  the  boundaries  of  the  zone, 
the  deposits  filling  them  assuming  the  character  of  local  fissure  veins. 
It  is  possible  that  in  such  cases  the  fissuring  may  have  been  wholly 
secondary  to  the  primary  fracturing — cracks  opening  within  the  already 
partly  mineralized  body  of  rock.  In  other  cases  the  original  movement 
may  have  resulted  in  a  fissure,  the  present  brecciation  being  due  to 
included  fragments  of  the  country  rock,  themselves,  perhaps,  minutely 
fractured. 

In  the  mineralization  of  the  fractured,  ore-bearing  zones,  the  metallic 
sulphides  occur  distributed  through  the  interstitial  filling,  or  accumu- 
lated in  thin  layers  along  partings  in  the  vein  material,  or,  again,  in 
massive  bodies  of  ore,  1  to  20  feet  thick,  on  one  or  the  other  of  the  walls, 
in  the  interior  of  the  zone,  or  occupying  its  entire  width.  Many 
instances  of  the  replacement  of  limestone  within  the  fractured  zone  by 
metallic  compounds  occur.  Such  might  also  have  been  the  case  with 
the  heavier  bodies  of  ore.  On  the  other  hand,  the  heavier  bodies  may 
have  been  deposited  from  solution  in  the  fissures  opened  in  the  fractured 
zone.  Oftentimes,  where  there  is  a  vein  of  nearly  solid  ore,  or  at  least 
a  body  of  highly  mineralized  rock  in  a  fractured  zone,  the  portions  of 
the  zone  on  either  side,  if  the  body  is  in  the  interior,  may  also  be  min- 
eralized in  paying  amount  and  the  adjacent  country  itself  may  carry  a 
small  percentage  of  ore. 

Of  the  class  of  ore  deposits  occurring  in  beds  of  limestone  a  single 
instance  was  observed  by  the  writer,  but  this  form  of  deposit  may  not 
be  uncommon  in  the  Wood  Eiver  district,  where  like  conditions  are 
maintained  over  such  an  extended  area.  The  deposit  bore  considerable 
resemblance  to  the  deposits  at  Leadville,  in  the  "blue  limestone"  of 
sub-Carboniferous  age.  It  was  not,  however,  accompanied  by  eruptives, 
and  this  absence  of  eruptives  in  connection  with  ore  bodies  prevails  in 
the  Wood  Itiver  region ;  the  source  of  the  mineralization  is  therefore 
quite  different  in  the  two  localities.  In  the  deposit  observed  by  the 
writer  there  is  a  mineralized  zone  of  limestone  with  an  included  vein 
of  ore.  Where  seen  the  total  width  of  the  metalliferous  zone  was  5  or 
6  feet,  and  that  of  the  included  vein  3  inches  to  1 J  feet.  Both  are  said 
to  have  reached  greater  width  in  other  portions  of  the  mine,  12  feet  of 
galena  being  reported  at  one  point.  The  country  rock  is  the  dark-gray 
calcareous  shale  and  limestone,  the  gaugue,  quartz  with  a  little  calcite, 
and  here  and  there  portions  of  shale.  The  ore  bodies  in  veins  of  this 
nature  are  said  to  be  nearly  independent  of  one  another,  and  appear  to 
have  originated  in  the  mineralization  of  larger  or  smaller  portions  of 
limestone  which  were  within  easiest  access  of  the  mineral-bearing  solu- 
tions. It  is  quite  reasonable  to  suppose  that  in  a  region  of  this  kind 
in  the  folding  which  the  strata  have  undergone  bodies  of  limestone, 
parts  of  an  entire  bed,  may  have  been  reticulated  with  cracks  without 
displacement  of  the  fragments,  and  that  through  such  bodies  solutions 


270  GEOLOGICAL    RECONNAISSANCE   ACROSS   IDAHO. 

found  an  easy  passage.  In  a  region  of  such  fracturing  and  so  many 
channels  for  the  flow  of  mineral  solutions,  any  of  the  limestones  may 
become  locally  ore-bearing.  For  the  preference  of  the  limestones  of  the 
shaly  series  over  the  massive  gray  limestone  of  the  region,  no  explana- 
tion was  arrived  at ;  the  suggestion  occurs  that  it  may  be  due  to  original 
composition. 

The  class  of  veins  which  may  be  considered  as  having  had  their 
origin  in  fissures — fissure  veins — occurs  both  in  the  great  calcareous 
slate  and  limestone  series  of  the  Wood  Kiver  district  and  in  the  granite 
just  beyond.  In  the  slates  they  are  doubtless  numerous.  They  are 
here  distinguished  by  their  strike  and  dip,  which  is  at  variance  with 
that  of  the  slaty  beds,  by  slickensides,  by  selvages  of  clay,  by  occa- 
sional included  fragments  of  the  country,  and  by  the  character  of  the 
filling,  which  is  a  heavy  deposit  of  quartz  in  one  or  more  layers,  with 
or  without  accessory  minerals  like  calcite,  the  metallic  contents  being 
deposited  in  masses  along  the  planes  or  disseminated  throughout  the 
ledge  matter.  Moreover,  as  the  vein  crosses  the  stratification  of  a 
series  of  thin-bedded  rocks,  the  nature  of  the  country  is  constantly, 
changing,  both  on  the  course  of  the  vein  and  in  depth.  Original  fis- 
sure veins  in  slates  contain  more  or  less  debris  from  the  country,  but 
are  usually  easily  distinguished  from  the  zones  of  fracture  and  their 
included  local  fissures. 

Near  Doniphan,  on  the  head  of  the  east  fork  of  Camp  Greek,  south- 
west of  the  great  area  of  calcareous  slates,  and  in  the  region  of  gran- 
ite, is  a  prominent  quartz  ledge  having  a  general  course  of  N.  30°  to  35° 
W.  and  extending  with  interruptions  for  3  or  4  miles  across  the  country, 
Doniphan  being  near  its  southeastern  terminus.  The  granite  is  some- 
what foliated,  and,  so  far  as  observed,  the  planes  have  a  prevailing 
strike  N.  25°  W.  and  a  dip  southwest.  The  ledge  presents  some  minor 
curvatures  of  strike,  and  the  dip  also  varies  a  little  from  point  to  point, 
but  is  in  general  about  45°  NE.  The  vein  doubtless  belongs  to  the  class 
of  true  fissures — a  view  strengthened  by  slickensides,  clay  selvages, 
and  fragments  of  the  country  in  the  ledge  matter.  There  are  several 
locations  upon  the  ledge,  but  the  Gamas  No.  2,  at  the  southeastern  end, 
is  the  only  mine  of  present  importance'and  is  said  to  have  been  a  good 
producer  in  earlier  times.  The  stopes  would  confirm  this.  It  is  now 
about  being  reopened.  The  width  of  the  ledge  varies,  the  maximum 
observed  being  between  6  and  10  feet.  The  quartz  is  banded,  coarsely 
crystalline  and  showing  little  mineral,  or  fine-grained  and  mineralized 
to  considerable  amount.  At  a  single  point  a  bit  of  limestone  resem- 
bling that  of  the  shaly  series  was  discovered  adjacent  to  the  ledge,  but 
its  presence  was  inexplicable.  Its  appearance,  however,  was  that  of  an 
intimate  part  of  the  rock,  not  an  included  fragment.  The  ore  occurs  in 
shoots  of  greater  or  less  size,  the  mineral  contents  being  either  dissem- 
inated in  particles  throughout  the  entire  width  of  the  ledge,  concen- 
trated in  bunches  on  the  walls  or  in  the  interior,  or  confined  to  certain 
benches. 


ELDHIDOE.]  SILVER    CITY    DISTRICT.  271 

The  ores  of  the  Wood  River  district  are  lead-silver  and  gold.  The 
former  is  practically  confined  to  the  great  series  of  calcareous  shales, 
the  latter  to  the  granite. 

Associated  with  the  galena  of  the  first  class  of  ores  are  blende,  pyrite, 
arsenical  pyrites,  gray  copper,  erubescite,  and  occasionally  native  silver. 
Blende  and  common  pyrite  are  the  most  widely  occurring  and  abundant 
associates;  the  others  are  comparatively  rare.  Carbonates  usually 
occur  near  the  surface.  The  galena  is  fine  to  coarsely  crystalline,  and 
occurs  disseminated,  bunched,  or  in  extended  bodies.  It  often  shows 
striation  resulting  from  twinning.  The  blende  occurs  either  through 
the  galena  or  in  distinct  portions  of  the  vein  with  but  slight  admixture 
of  galena.  The  pyrite  may  occur  in  the  same  manner  as  the  blende, 
but  is  of  wider  and  more  universal  distribution  than  any  of  the  other 
minerals,  and  moreover  js  frequently  found  in  the  country  adjacent  to 
the  ore  bodies.  No  examination  as  to  the  contents  of  the  ores  was 
attempted,  but  they  are  said  to  run  as  high  as  160  ounces  of  silver  and 
70  per  cent  of  lead  to  the  ton,  with  an  average  considerably  below  this. 

The  gold  ores  occur  in  the  veins  in  granite.  They  are  in  part  free- 
milling,  in  part  smelting.  The  minerals  associated  with  the  gold  are 
pyrite  and  chalcopyrite,  with  their  alteration  products,  and  a  very 
small  amount  of  galena,  probably  argentiferous,  as  silver  is  found  in 
such  ores  on  examination.  The  milling  ores  are  said  to  run  from  $8  to 
$15  per  ton  in  free  gold,  the  assays  indicating  a  value  of  $20  in  this 
metal. 

THE   SILVER   CITY  DISTRICT. 

This  is  located  on  the  southwestern  slope  of  the  Owyhee  Range,  about 
the  head  waters  of  the  Jordan  River,  a  tributary  of  the  Owyhee;  the 
district  extends,  however,  to  the  heads  of  other  streams  in  the  vicinity, 
and  altogether  occupies  an  area  8  or  10  miles  square.  The  two  leading 
towns  are  Silver  City,  at  the  head  of  Jordan  Creek,  and  De  Lainar,  7 
miles  below,  to  the  west.  The  Jordan  Valley  and  its  tributaries  are 
sharply  eroded  to  depths  of  2,000  to  3,000  feet  below  the  crests  of  the 
range  and  its  spurs.  The  water  supply  is  light.  Timber  is  scarce.  The 
district  is  50  miles  from  the  railroad,  Nampa  on  the  Oregon  Short  Line 
being  its  station.  An  excellent  wagon  road  over  the  mountain  and 
across  the  Snake  Valley  connects  the  two  points. 

The  Owyhee  Range,  in  which  are  situated  the  two  important  mining 
camps  of  Silver  City  and  Be  Lamar,  is  primarily  a  granite  range,  in 
later  times  cut  by  rhyolite,  diabase,  and  basalt. 

The  two  leading  mines  of  the  district  are  the  De  Lamar  at  De  Lainar, 
and  the  Black  Jack,  1J  miles  west  of  Silver  City.  Both  are  on  clearly 
defined  fissure  veins  in  rhyolite.  There  is,  however,  in  the  Black  Jack 
an  associated  rock,  diabase,  and  on  the  mountain  side  below  this  mine 
basalt  occurs.  The  rhyolites  are  several  miles  in  extent,  but  whether 
the  two  mines  are  on  the  same  body  was  not  ascertained.  The  rhyolite 
in  a  decomposed  state  is  sometimes  mistaken  for  granite  by  the  miners. 
The  diabase,  which  is  called  "trachyte "by  the  miners,  occurs  locally 


272 


GEOLOGICAL   RECONNAISSANCE   ACROSS   IDAHO. 


on  one  side  of  the  vein  in  the  Black  Jack  mine,  and  in  the  Trade  Dol- 
lar, to  the  east  of  the  Black  Jack,  it  is  said  to  form  both  walls.  This 
rock  is  younger  than  the  rhyolite  with  which  it  is  associated. 

The  veins  trend  between  N.  10°  and  60°  W.,  varying  locally;  the 
prevailing  dip,  which  is  steep,  is  west  or  south.  Fracture  planes  cross 
the  veins  at  greater  or  less  angles,  but  examination  was  not  in  suffi- 
cient detail  to  determine  a  system.  The  width  of  the  veins  is  from  an 
inch  or  two  up  to  10  feet  or  more.  The  vein  matter  is  quartz,  banded 
or  massive,  coarsely  or  finely  crystallized.  The  quartz  locally  shows  a 
brecciated  structure,  coarse  or  fine,  indicating  movements  since  the 
formation  of  the  veins;  the  fragments  are  often  somewhat  abraded 
and  surrounded  by  a  clay  gouge.  Not  infrequently,  also,  included 


114*50' 


16*30' 


Flo.  41.— Sketch-map  of  Silver  City. 

fragments  of  the  country  rock  are  found,  either  as  horses  or  in  small, 
angular  pieces.  Talc,  so  called,  is  of  common  occurrence  in  certain 
localities  in  the  mines,  even  in  the  interior  of  the  ledge.  Besides  this, 
there  is  the  usual  selvage  on  the  walls,  attaining  locally  a  considerable 
thickness. 

The  ores  are  gold  and  silver.  Both  metals  occur  native,  but  the 
silver  is  found  in  large  proportion  as  argentite.  The  chloride  is  also 
present.  Pyrite,  auriferous  and  nonauriferous,  occurs.  The  talc  in  the 
veins  is  a  remarkably  productive  class  of  ore,  carrying  a  large  amount 
of  argentite.  The  ores  are  generally  of  very  high  grade. 


BLDRIDQB.]  PLACERS.  273 

PLACERS. 

The  mills  of  the  companies  are  well  equipped  and  are  kept  running 
night  and  day  a  greater  part  of  the  year. 

The  placers  on  the  route  of  reconnaissance  are  chiefly  gravel  bars 
along  stream  bottoms.  From  reports  some  of  them  have  yielded  fabu- 
lously in  the  past,  but  at  present  work  is  prosecuted  only  by  a  few 
scattered  Chinese  laborers.  The  placer  gravels  encountered  were  all 
on  streams  that  had  their  sources  in  the  gray  granite  or  in  the  crystal- 
line schists  which  lie  upon  this.  Indeed  there  are  apparently  but  few 
streams  rising  in  these  terranes  that  do  not  at  some  point  carry  pay 
gravel  or  have  not  already  produced  more  or  less  gold.  Among  the 
more  notable  instances  are  the  bars  of  the  main  Boise  and  its  tribu- 
taries— South  Fork  below  Eocky  Bar,  and  Middle  Fork,  below  Atlanta; 
Loon  Creek,  entering  the  Middle  Fork  of  the  Salmon;  and  Yellow 
Jacket  Creek  below  the  mining  camp.  Leesburg,  on  Napias  Creek,  has 
been  the  center  of  a  great  placer  district  in  early  days,  and  at  Cali- 
fornia Bar,  3  miles  below,  a  company  is  now  preparing  to  work  a  sup- 
posed extensive  placer. 

Different  from  the  foregoing  class  of  placers  is  that  on  Kirtley 
Creek,  8  miles  east  of  Salmon  City,  within  2  miles  of  the  base  of  the 
Continental  Divide.  There  is  here,  indeed,  as  along  several  of  the 
streams  heading  in  this  range,  the  usual  river  and  creek  gravel  bars, 
carrying  a  greater  or  less  amount  of  gold,  but  in  the  bluifs  of  the 
creek  there  is  also  an  extensive  placer  probably  of  Tertiary  age,  which 
is  now  being  opened.  The  general  feature  of  this  deposit  is,  briefly,  a 
succession  of  Tertiary  gravels  and  sands,  which  possibly  correspond 
with  beds  farther  out  in  the  Salmon  Valley  that  from  paleobotanic 
evidence  are  Eocene  or  Miocene.  The  material  was  derived  from  the 
neighboring  range,  and  formed  a  shore  deposit  of  the  intermontane 
lake.  Their  north-aud-south  extent  was  not  learned,  but  it  may  be 
several  miles.  The  gravels  may  not  everywhere  be  auriferous,  however. 

The  deposit  as  exposed  consists,  from  base  up,  first,  of  a  series  of 
conglomerates  and  sandstones,  about  equally  divided,  cut  to  a  depth 
of  15  feet,  and  found  to  be  auriferous — in.  paying  amount,  it  is  said, 
under  a  sufficient  supply  of  water.  Upon  this  series  is  a  very  white, 
thin-bedded  sandstone  but  a  few  feet  in  thickness,  succeeded  by  two 
other  layers,  of  white  and  yellow  sandstones,  5  and  3  feet  thick 
respectively,  which  are  leaf- bearing;  these  are  called  by  the  miners 
bed-rock,  as  they  form  the  foundation  for  the  overlying  gravels,  which 
are  uuconformable  and  said  to  be  fairly  rich,  especially  near  the  bottom. 
The  overlying  gravels  are  fully  20  feet  thick  when  present  in  their 
entirety,  and  are  in  turn  succeeded  by  30  feet  of  interbedded  sand- 
stone and  conglomerate,  the  sandstones  predominating,  and  all  carrying 
more  or  less  gold.  This  completes  the  succession  of  beds  of  supposed 
Tertiary  age.  The  entire  series,  both  above  and  below  the  line  of  nou- 
16  GEOL,  PT  2 18 


274      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

conformity,  is  flexed,  a  westerly  dip  of  .15°  to  45°  prevailing.  This 
bending  of  the  strata  indicates  that  if  the  portion  above  the  break  is 
not  of  the  same  age  as  that  below,  it  is  at  least  earlier  than  Pleistocene. 

Eestiug  upon  any  of  the  foregoing  beds,  according  to  tlieir  exposure 
in  the  past,  is  a  Becent  gravel,  derived  in  part  from  that  below,  iu  part, 
perhaps,  from  the  present  mountain  slopes.  This  gravel  is  said  to  be 
the  richest  of  all,  the  result  of  a  natural  concentration  of  the  earlier 
auriferous  gravels.  The  gravel,  both  Tertiary  and  Eecent,  is  of  quartz- 
ite  or  quartzite-schist  debris;  the  Tertiary  portion  is  usually  tightly 
cemented  and  somewhat  difficult  to  hydraulic.  The  gold  contents  were 
not  ascertained,  but  elaborate  preparations  are  being  made  for  mining. 

The  present  supply  of  water  is  from  storage  reservoirs  in  the  moun- 
tains, and  is  insufficient  for  constant  work.  It  is  proposed  to  take 
water  from  the  Lemhi  Eiver,  which  would  probably  obviate  this  incon- 
venience. 

COAL. 

Two  coal  areas  were  encountered  on  the  reconnaissance,  one  about 
Salmon  City,  the  other  at  Horseshoe  Bend  .on  the  Payette,  28  miles 
north  of  Boise.  Both  areas  are  probably  Tertiary — Eocene  or  Mio- 
cene— confirmatory  plant  remains  having  been  found  in  the  beds  at 
Salmon  City  and  the  series  on  the  Payette  bearing  a  close  general 
resemblance  to  these  in  composition,  in  manner  of  occurrence,  and  in 
folding.  The  coal  occurs  in  small  seams  in  sandstones,  shale  or  con- 
glomerate lying  within  a  few  feet. 

SALMON   CITY   VALLEY. 

The  only  opening  in  this  valley  is  a  drift  20  feet  in  length  in  the  east 
bluff  of  the  Salmon  Eiver  a  mile  below  the  city.  There  are  here  several 
thin  streaks  of  lignite — the  thickest,  6  inches — in  5  feet  of  carbonaceous 
slate  and  sandstone.  The  lignite  has  a  dead-brown  appearance,  and 
its  woody  structure  is  plainly  visible;  the  ash  is  high,  as  are  probably 
also  the  water  contents.  It  is  valueless  so  far  as  exposed.  The  meas- 
ures which  carry  this  lignite  underlie  a  large  portion  of  the  valley  about 
Salmon  City,  and  at  several  points  -show  thin  beds  of  carbonaceous 
shale,  indicating  the  former  presence  of  plant  life  and  a  tendency  to 
the  formation  of  coal ;  but  the  conditions  seem  not  to  have  been  those 
requisite  for  the  development  of  a  coal  of  economic  value. 

HORSESHOE   BEND,  VALLEY  OF   THE   PAYETTE. 

The  valley  of  the  Payette  at  Horseshoe  Bend  has  a  northeast  trend, 
is  about  9  miles  long  by  3  across,  and  is  divided  midway  its  length  by 
the  axis  of  an  anticlinal  arch  having  a  general  northwest  direction. 
•Coal  occurs  in  both  the  northern  and  southern  halves  of  the  valley.  The 
northern  prospects  were  not  visited,  being  reported  but  slightly  opened 
and  in  unsatisfactory  condition  for  examination.  The  only  southern 


COAL   AND   AGRICULTURE.  275 

opening  at  present  is  a  surface  cut  and  short  tunnel  on  what  is  known 
as  the  Hob  vein,  of  which  the  following  is  said  to  be  a  section,  though 
only  the  upper  coal  was  visible  at  the  time  of  visit: 

Section  of  the  Bob  coal  vein,  Horsehoe  Bend,  valley  of  the  Payette. 

Sandstone  and  shale roof. 

Coal 1ft.    6  in. 

Slate 1  ft.    Oin. 

Coal .' Oft.  10  in. 

Slate 1  It.    0  in. 

Coal Oft.  10  in. 

Clay floor. 

The  strike  of  the  vein  at  the  exposure  is  northwest;  tie  dip,  SW.  5° 
to  10°.  A  short  distance  south  an  eruptive  a  few  hundred  feet  wide 
cuts  the  coal.  Beyond  this  the  measures  again  appear,  but  are  of  lim- 
ited extent,  owing  to  the  proximity  of  the  granite  of  the  Boise  Range, 
or  of  a  second  eruptive.  Northwest  the  coal  bed  may  extend  for  a  con- 
siderable distance  before  reaching  the  edge  of  the  valley,  the  western 
wall  of  which  is  granite. 

The  coal  is  black  and  comparatively  hard.  Following  is  the  analysis 
of  a  sample  taken  from  a  small  pile  on  the  dump,  which  is  said  to  have 
come  from  a  tunnel  inaccessible  at  the  time  of  visit. 

Analysis  of  coal  from  Horseshoe  liend,  valley  of  the  Payette. 

Moisture 4. 84 

Volatile  matter 36. 23 

Fixed  carbon 54. 55 

Ash  . .  4. 38 


Total '100.00 

The  excellent  quality  of  the  coal  in  the  present  instance  may  be 
explained  by  its  proximity  to  an  eruptive.  It  is  questionable  whether 
this  grade  would  be  maintained  over  the  entire  valley. 

AGRICULTURE. 

In  crossing  and  recrossing  the  State  the  following  facts  in  regard  to 
agricultural  possibilities  were  impressed  upon  the  writer:  Much  of 
Idaho  is  a  rugged  mass  of  mountains,  and  but  little  area  is  available 
for  farming  in  comparison  with  that  of  many  other  Western  States. 
The  intermontane  valleys  are  few,  but  are  very  fertile  and  well  adapted 
to  hay,  grain,  vegetables,  and  fruits.  Berries,  apples,  peaches,  plums, 
and  prunes,  all,  in  one  locality  or  another,  attain  remarkable  perfection. 
In  the  Boise  Valley  and  the  country  adjacent  the  prune,  plum,  and 
peach  industries  have  been  successfully  started  and  promise  enormous 
advances  in  the  next  few  years.  The  apple  industry  is  also  large,  but 
in  this  there  is  apparently  a  choice  of  location.  In  certain  portions  of 
the  State  a  worm  seriously  deteriorates  the  crop.  In  the  Salmon  City 

1  No  coke. 


276      GEOLOGICAL  RECONNAISSANCE  ACROSS  IDAHO. 

Valley,  however,  it  has  not  yet  made  its  appearance,  and  the  apples 
grown  here,  upon  the  talus  slopes  at  the  foot  of  the  mountains,  and  in 
the  higher  valleys,  are  the  finest  in  flavor  and  the  largest  in  size  the 
writer  has  ever  seen.  Berries  and  vegetables  may  be  grown  with  great 
ease  in  all  the  valleys  of  the  State.  The  area  for  large  farming  opera- 
tions, the  growing  of  hay,  grain,  etc.,  is  at  present  small,  but  is  capable 
of  great  increase  by  the  irrigation  of  the  many  thousand  acres  that  can 
be  selected  on  the  plains  of  the  Snake  Eiver.  That  such  lands  are 
suitable  for  cultivation  under  irrigation  has  already  been  proved  by  the 
successful  farms  and  orchards  now  in  operation.  Such  irrigation,  how- 
ever, can  be  accomplished  only  on  a  large  scale,  by  heavy  outlays  of 
capital.  Artesian  water  is  possible  at  many  points,  the  porosity  of  the 
strata,  both  in  the  Snake  Valley  and  in  the  intermontane  depressions, 
rendering  a  large  subterranean  circulation  quite  probable.  Hot 
springs  abound,  remarkable  for  their  number,  distribution,  and  size. 
They  are  already  utilized  at  Boise  in  the  municipal  economy. 


• 


^^^^H 


:W:g::B; 


